Flank Pain: Approaches in Emergency Diagnosis and Management
Taline Kilaghbian MD*, Sara Wong MD*, Laura E. Crocitto MD+, Sean O. Henderson MD*
*Department of Emergency Medicine
Keck School of Medicine
University of Southern California
Los Angeles, CA 90033
+Department of Urology
City of Hope
Duarte, California 91010
Flank Pain: Approaches in Emergency Diagnosis and Management
Flank pain is a common ED complaint with acute ureteral obstruction being the most common cause. However, because of the complex innervation pattern of the flank, pain can arise from a number of different organ systems.  Multiple studies have shown that after using noncontrast CT scans in patients who were thought to have renal colic, about 10% are ultimately given an alternate diagnosis. [2,3]
In order to achieve the correct diagnosis and to implement the correct management, a number of laboratory and diagnostic tests must be utilized along with the standard criteria of age, gender, history, and physical exam.
An understanding of the innervation pattern of the abdomen and flank is essential to understanding the complexity of flank pain. The “flank” describes the posterior portion of the body between the ribs and the ilium. Pain in this area arises from a myriad of locations ranging from the skin and superficial muscles to deeper abdominal structures. Most intra-abdominal structures are innervated by visceral fibers and pain is felt secondary to distention or muscle contraction. The pain is usually poorly localized and associated with nausea and autonomic symptoms.
Visceral pain may radiate or may also be referred in characteristic patterns. The dermatomal rule implies that visceral pain is referred to a structure that developed from the same embryonic segment or dermatome as the structure in which the pain originates. A classic example is the pain experienced by ureteral colic in men that is classically described as flank pain radiating to the ipsilateral testicle. As the testicle migrates with its nerve supply from the same primitive urogenital ridge as that which the kidney and ureter develops, this radiation pattern is easily understandable. The lack of a consistent pain pattern is further explained by the convergence of somatic and visceral afferents onto spinothalamic neurons. Consequently, pain originating from these structures is poorly localized and nonspecific due to the overlap of nerves supplying the abdominal and pelvic structures. [1,4]
The differential diagnosis of flank pain is vast. All organ systems starting from the skin and proceeding inward must be considered. The following, Table 1, is a list of the differential diagnosis: [1,3,9,10,11]
The differential diagnosis of flank pain is vast. All organ systems starting from the skin and proceeding inward must be considered. The following, Table 1, is a list of the differential diagnosis: [1,3,9,10,11]
• Abdominal aortic aneurysm
• Aortic dissection
• Superior mesenteric artery occlusion
• Acute myocardial infarction
• Renal artery occlusion or dissection
• Renal Cell Carcinoma
• Ectopic pregnancy
• Psoas abscess or hematoma
• Retroperitoneal mass
• Internal hernia
• Pulmonary embolus
• Renal colic
• Adnexal mass/torsion/hemorrhage
• Choledocholithiasis/cholecystitis/biliary colic
• Pelvic inflammatory disease
• Fitz-Hugh Curtis syndrome
• Stress fractures of ribs
• Musculoskeletal pain
• Herpes Zoster
The focus of the pre-hospital care setting should be on initial resuscitation and stabilization of the patient being transported for definitive medical care. The field vital signs will be the greatest parameter guiding paramedics in the management of the flank pain patient. A hypotensive, tachycardic patient with flank pain may need IV access and aggressive fluid resuscitation en route to the hospital while a normotensive, stable patient may simply require analgesia and pain control in the pre-hospital setting. However, time to transport should not be significantly delayed in an effort to obtain IV access or provide non-critical care.
Obtaining a thorough history is an important component in the initial evaluation of the patient with flank pain. Factors helpful in steering the evaluation in the proper direction include: the location of the pain, rapidity of onset, quality, radiation, duration, constant versus intermittent symptoms, alleviating versus aggravating factors, and associated symptoms such as fever, vomiting, hematuria, cough, and menstrual history. Underlying medical illnesses, such as hypertension, history of nephrolithiasis, recurrent UTI, diverticulosis, Crohn’s disease, hyperparathyroidism, sickle cell disease or HIV may aid in narrowing the differential. Other important factors include the patient’s age; gender; history of substance use; current medication use; trauma; immobilization; immunosuppression; and a history of similar symptomatic episodes.
It is imperative to remember, however, that there is a tremendous amount of variation and overlap in the way different illnesses present. Therefore, a history alone, or even in conjunction with a physical exam, is often not enough to make a definitive diagnosis. Features of the history that will aid the emergency physician (EP) toward a diagnosis will be discussed later in this paper in the context of specific differential diagnoses.
The physical exam begins with the vital signs with red flags including tachycardia, hypotension, and fever. The EP must never assume that hypotension is due to volume loss from vomiting or is vasovagal in origin. In Marston’s study of AAAs, 71% of correctly diagnosed patients and 57% of misdiagnosed patients were in shock at presentation.  In patients who are in pain, tachycardia may be present in varying degrees. It may also accompany fever and hypotension and, therefore, is less specific.
There are many initial subtle general exam findings that will help direct the physician to a diagnosis. The febrile patient who enters the room with a new limp or holds the hip in flexion or external rotation may have an iliopsoas abscess or appendicitis. The patient with renal colic tends to be in constant movement, whereas the patient with pyelonephritis or peritonitis tends to lie still to avoid further irritation. The physical exam should include an initial pulmonary and cardiac exam, as pathology in both organ systems can present as flank pain. With regard to the abdominal exam, there are a number of factors that will aid the clinician in narrowing the differential diagnoses; the location of abdominal tenderness, presence of abdominal distention, distinction between stable or worsening pain, peritoneal signs, and presence of a pulsatile mass such as in AAA. Flank pain in the presence of abdominal systolic and diastolic bruits over the flank may indicate renal artery occlusion, aneurysm, or arteriovenous malformation. It is important to examine the patient for the quality and presence of peripheral pulses, which may be altered in aortic dissection; costovertebral angle tenderness versus muscular tenderness; muscle spasm; and perform a skin exam looking for rashes, as may be seen with Herpes Zoster. Flank or periumbilical ecchymosis, Grey-Turner’s sign and Cullen’s sign respectively, occurs in only 1-3% of acute pancreatitis and is not diagnostic. It may be secondary to direct blunt or penetrating trauma, perforated duodenal ulcer, liver disease, or other intraabdominal or retroperitoneal bleeding.  The EP should palpate for crepitus over the chest wall or abdomen secondary to fractured ribs and perform a genital or pelvic exam, when indicated, as it is also an important aspect of the physical exam. Vaginal bleeding and/or a palpable adnexal mass allude to possible ectopic pregnancy or adnexal abscess as the source of pain. Pain on rectal palpation may be indicative of diverticulitis in the patient with flank pain.
As noted earlier, flank pain is a complex complaint with a great deal of overlap in symptomatology and physical findings, making a clinical diagnosis both very challenging and perilous.
In depth with some of the differential diagnoses:
Renal colic is the most common cause of flank pain that presents to the emergency department.  About 12% of the US population will develop nephrolithiasis by the age of 70 and about 3-5% will experience renal colic in their lifetime, with the global incidence of the disease continuing to rise in recent years. [7,9] The peak age of presentation of renal colic in the United States is between the ages of 40 and 49; initial presentation is relatively uncommon before the age of 15 or after the age of 50. There is a male to female ratio of 3:1 and a recurrence rate of about 50% within 10 years. [1,7,8,9]
The distinction between nephrolithiasis and renal colic must be made clear. The simple presence of nephrolithiasis does not explain the presence of symptoms. It is only after passage of the stone out of the renal pelvis and into the collecting system that the muscular spasm felt as renal colic will occur. In fact, a large stone in the kidney is less likely to cause symptoms as it is unable to make such a passage.
Many solutes in urine are found in a supersaturated state. Therefore small changes in solution characteristics, such as solute concentration, pH, or presence of ionic molecules may lead to crystal formation. These crystals then serve as a nidus for stone formation. Stone composition is as follows: [9,10]
• Calcium oxalate (monohydrate or dihydrate) and calcium phosphate stones comprise 75-80% of stones
• Phosphate containing stones comprise 15% with struvite being the most common
• Uric acid stones comprise 5-10% of stones; they are the most radiolucent
• Cystine stones comprise 1% of stones
• Miscellaneous stones include xanthine, triamterene, and indinavir.
A few investigators have attempted to identify historical, physical, and laboratory criteria that may aid the EP in reaching a clinical diagnosis of ureteral colic. In a Finnish study of 1,333 patients presenting with abdominal pain, Eskelinen et al found that acute abdominal pain (<12 hours duration), normal appetite combined with hematuria (>10 erythrocytes), flank or renal tenderness had a sensitivity of 84% and specificity of 99% for renal colic. In this study however, the diagnosis of renal colic was made by plain abdominal radiograph, intravenous urography (IVU), laboratory tests, or clinically. Because no gold standard was used, it is unclear what clinical criteria was used to make the diagnoses and therefore, the validity of the study. Moreover, they recommended those criteria only as an aid to decision making. 
Location of the stone appears to have an importance in the pain pattern. Renal colic pain often begins as a vague pain and as the stone progresses, tends to travel medial and caudally. Specifically located obstructive stones presented with the following symptoms: kidney- vague flank pain and hematuria; proximal ureter- flank and upper abdominal pain; mid ureter- flank and anterior abdominal pain; and distal ureter- flank and anterior abdominal pain, dysuria and increase in urinary frequency.  Still, determining renal colic to be the etiology of flank pain remains difficult. There remains no physical symptoms specific to renal colic. A retrospective study by Luchs et al looking for a correlation between renal colic and hematuria showed that only 52% of patients had stones confirmed by noncontrast CT.  A similar study by Elton et al attempted to make a prediction rule for the diagnosis of ureteral colic using a derivation group of 203 retrospectively reviewed charts and a validation group of 72 prospective patients, concluding that acute onset of flank pain and hematuria had a 90% probability of being ureteral colic. Further conclusions reported an increase in probability to 98.5% if a KUB was positive, indicating that emergent intravenous pyelogram (IVP) could be deferred. They recommended that the results be confirmed in another emergency department. 
Abdominal aortic aneurysm(AAA):
Ruptured AAA is a potentially catastrophic event. Incidence varies by geographic location and ranges between 3-8%. [5,16,17] The risk for AAA rupture is 3 times higher in women and in the 13th leading cause of death in the US. Sixty-two percent of patients die before reaching the hospital; mortality is 15-70% with surgery and 100% without. [5,18] In a 15-year retrospective study of 152 patients, Marston et al reported that 30% of patients with ruptured AAA were misdiagnosed and the most common misdiagnosis was renal colic. In the majority of these individuals, abdominal pain (76%) and back pain (54%) had been the most common presenting symptoms. The classic triad of abdominal pain, back pain, and pulsatile abdominal mass was found in only 26% of the individuals overall. What is difficult to explain is that 26% of misdiagnosed patients had a pulsatile mass present on physical exam and 57% of these misdiagnosed patients presented to the emergency department in shock. Interestingly, in one retrospective study there was no difference in mortality between the correctly diagnosed and the misdiagnosed patients, possibly because these patients were a more stable subset with a more contained bleed. 
Explanations for an absence of a palpable mass in the setting of a significant AAA include abdominal distention due to hemorrhage, a low systemic blood pressure, and obesity (especially when abdominal girth exceeds 100cm). [5,16] Similarly another study indicated that palpation detected only about 39% of all AAAs, a positive predictive value of 43%, with sensitivity only improving as aneurysm size increases, except in cases of obesity and rupture. 
Population-based studies have found that the risk of AAA rupture increases when diameter exceeds 5 cm, however, small aneurysms of even 3 cm have been known to rupture. [5,19] One cohort study showed the greatest risk for rupture in the 6.5-7.9cm range, therefore elective surgery for asymptomatic patients is recommended when the aneurysm size is between 5 and 6cm. [19,20] Similar studies by Davies et al have found that AAA greater than 6cm had a 3.7% rate of rupture per year and those above 4.25cm had a death, rupture or dissection rate of 20% per year. Any diameter greater than 6mm increased risk 27-fold. [21,22]
Further clouding the picture, a small retrospective study done by Pomper et al found 87% of patients with a ruptured AAA presented with hematuria. The finding of hematuria can be explained by an aortocaval fistula formation secondary to a ruptured aortic aneurysm. Despite this distracting symptom, there was no evidence of delay in treatment, an effect perhaps of the small sample size. [23,24]
A key point to note is that most ruptures occur in patients who have no history of AAA. Individuals who are aware of the presence of an abdominal aneurysm typically have surgery before the aneurysm reaches 6 cm in diameter. 
Acute renal artery occlusion (RAO):
Acute RAO may present with flank pain and is a well known mimic of renal colic. While the most common cause of RAO is blunt abdominal trauma, cardiac conditions, renal conditions, and hypercoagulable states are also significant causes. A limited list includes cardiac arrhythmias, valvular disease, atherosclerosis, polyarteritis nodosa, nephrotic syndrome, and polycystic kidney disease.  Despite this rather extensive differential, 90% of emboli originate from the heart and are associated with dilated cardiomyopathy, myocardial infarction, valve disease and atrial fibrillation. [26,27]
RAO is most common in the 6th and 7th decades of life with a presentation very similar to renal colic, including hematuria. Acute hypertension occurs in these patients only when there has been partial occlusion of the artery or sufficient collateral perfusion for the kidney has developed, allowing an ischemic state rather than complete infarction. 
Renal artery infarct is another rare, but often misdiagnosed, cause of flank pain and is often confused with nephrolithesis. A small study into found that of patients ultimately found to have acute renal artery infarct on CT, 80% had flank pain, elevated LDH and proteinuria. Of these patients half were misdiagnosed in the ED.  Symptoms appear to mimic nephrolithesis, i.e. flank pain, nausea, vomiting and possible fever, and it has also been found to present with hematuria and leukocytosis. Elevated LDH has been found to be the most sensitive sign of renal infarction. [27,29]
Renal artery or vein thrombosis may also mimic ureteral colic. Although rarer than embolic occlusion, it is associated with immunological renal diseases, hypercoagulable states, burns, and severe dehydration. 
Renal artery dissection is another form of occlusive disease that may present as a renal colic mimic, although there have been reports of silent presentations. A rare occurrence, it is more common in men, is often associated with fibromuscular dysplasia, and is associated with the development of malignant hypertension secondary to the resultant renal ischemia. A renal bruit may be present, however, there are no good studies on the frequency of this finding in the setting of dissection. Overall the prognosis is good, most likely due to the fact that renal ischemia is better tolerated in patients with a history of renal vascular disease secondary to the development of collaterals. [31,32,33,34,35] In such cases, renal tissue may remain viable and functional despite occlusion. 
Appendicitis is the most common acute abdominal surgical condition. It affects about 7% of the population with a peak incidence between 10 and 30 years of age. Diagnosis is primarily by history and physical exam, however computed tomography (CT) scan certainly has gained a prominent position in the diagnostic algorithm.
The presentation of appendicitis is typically abdominal pain, anorexia, and vomiting. The classic history of periumbilical pain migrating to the right lower quadrant is present in about half the patients.  Twenty to 30 percent of patients will have atypical presentations and laboratory findings.  As the appendix has no fixed position, presentation may vary based on its location. The appendix has a retroperitoneal location in 65% of patients and may descend into the iliac fossa in 31% of patients. There is no good data on how often appendicitis presents as flank pain.
In appendicitis, physical exam usually illustrates right lower quadrant tenderness and/or peritoneal signs with rebound or percussion tenderness. Retrocecal appendicitis may cause psoas irritation and present as flank pain.  In a retrospective study of patients with retrocecal appendicitis, 24% presented with right flank pain.  Diagnosis is especially challenging in female and pregnant patients because of the multitude of gynecological processes that can mimic appendicitis and because the gravid uterus can displace the appendix.
Defined as a necrotizing infection of the renal parenchyma, emphysematous pyelonephritis (EPN) is relatively rare with about 130 cases reported in the medical literature by 1996.  It is diagnosed by the presence of gas within the renal parenchyma and has an overall mortality rate of 40%. [40,41] There are two types of the infection: type 1 involves renal parenchyma damage while type 2 involves gas within the collecting system of the kidney. Type 1 has been found to have a higher mortality (69%) than type 2 (18%).  Although the full pathogenesis of the disease is unknown, it is suggested that the kidney provides a medium for bacteria to ferment sugars into carbon dioxide and produce gas, hence why the disease is associated with diabetes. [40,43] Seventy to 90% of cases occur in diabetics while approximately 15% have no previous history of diabetes.  In one large cohort of patients, the most common causative organisms were Escherichia coli and Klebsiella pneumoniae with some 22% of patients having urinary tract obstruction on the affected side.  Obstruction is almost universally present in non-diabetics. 
In a retrospective study of 48 patients with emphysematous pyelonephritis, Huang and Tseng showed that 79% of their patients presented with fever, 71% with flank/abdomen/or back pain, 35% with acute renal function impairment, 29% in shock, 19% with disturbance in consciousness, 17% with nausea/vomiting, and 13% with dyspnea.  As signs and symptoms mimic pyelonephritis, a poor response to antibiotics in a diabetic patient or septic/ toxic appearing patients should trigger an investigation into emphysematous pyelonephritis. The current average time to diagnosis of EPN is 2-7 days, with non-contrast CT (NCCT) scan as the diagnostic study of choice. [39,43]
Diverticula are outpouchings or herniations of the mucosa and submucosa through weaknesses in the musculature of the colonic wall where the nutrient vessels penetrate. The prevalence of diverticular disease varies by age ranging from 10% in patients less than 40 years of age and up to 50-66% in patients older than 80 years. The incidence of diverticulitis in the United States is increasing, with the greatest rise (82%) in incidence over 7 years in the 18-44 year age group.  In the West, ninety percent of diverticulitis cases occur in the distal colon, as opposed to Asian countries where right-sided disease predominates. Whether or not the etiology of right-sided disease is different remains uncertain. While the incidence of diverticulitis increases as Asians become Westernized, there is no evidence that anatomic location of their disease has changed. [46,47]
Diverticulitis implies an inflammation or infection and is generally the result of a single diverticular perforation. Subsequent abscesses of various sizes form.  Presenting symptoms are classically described as a left lower quadrant pain, however due to the complex innervation pattern, they may present with flank pain. Associated symptoms may include change in bowel habits, nausea, vomiting, anorexia, or dysuria secondary to a “sympathetic cystitis.” Depending on the stage of infection, guarding, localized tenderness, and peritoneal signs may be present. Numerous of studies document diverticular disease as an unexpected alternate diagnosis in patients evaluated for flank pain. [45,47]
Pyelonephritis refers to a bacterial infection of the kidney parenchyma and collecting system with interstitial inflammation. The typical signs and symptoms include flank pain, costovertebral angle tenderness, fever, nausea, vomiting, and dysuria. A retrospective chart review study found that 12% of patients with the above symptoms but lacking fever ultimately were given a different diagnosis. 
The evaluation of pyelonephritis usually includes laboratory tests such as electrolytes and renal function tests. A CBC may be misleading in that it is nonspecific. Patients with simple renal colic may have an elevated WBC because of demargination, while a patient with pyelonephritis may have a normal CBC. A urinalysis and urine microanalysis has the highest yield in these patients, with leukocyte esterase plus nitrite test both being positive in 75-84% in those with UTI and 90% of positive urine cultures in those with acute pyelonephritis. 
Bacteria that cause UTIs are fairly predictable. Seventy to 95% are caused by E. coli, with Staphylococcus saprophyticus as the cause in 5-20% of young women during the summer and fall months. Other pathogens include Proteus mirabilis, Klebsiella pneumonia, and Enterobacter species. Enterococci infection implies a mixed infection or underlying structural disease and Staphylococcus aureus suggests bacteremic spread. Atypical organisms such as Ureaplasma urealyticum and Mycoplasma hominis may also be causative but are difficult to grow in culture. 
Nephrolithiasis in Pregnancy:
The incidence of renal colic in pregnancy is similar to that in the general population with an average of one in 1500 pregnancies or 0.026-1.14% however the actual incidence of stones in pregnant woman is possibly much higher due to the fact that in the general population incidental stones are often found in asymptomatic patients. More common in the multiparas, episodes usually occur in the 2nd or 3rd trimester (52% and 22% respectively). [51,52] A multicenter study also found that while calcium oxalate stones are common in the non-pregnant population, calcium phosphate stones make up the majority of stones in pregnant women.  The gravid uterus may lead to altered pain perception, however one study found that 89% of pregnant renal colic patients presented with flank pain.  Confirmation of the diagnosis of calculi disease is imperative in these patients as other processes such as pyelonephritis, renal vein thrombosis, and renal rupture may all mimic these symptoms. [54,55]
Diagnosis is especially challenging in this patient population because of the gravid uterus and the various anatomic displacements it causes. Although ultrasound is the modality of choice, it lacks sensitivity and can be challenging because of the physiologic changes that occur with pregnancy. The sensitivity may be increased using resistive indexes, or in the case of distal stones, a transvaginal transducer.  In cases where the ultrasound is inadequate in making the correct diagnosis, a modified IVU can be done with prone positioning, low kilovoltage, and 3-4 films, adding up to 0.4-1.2 rads. [51,55,56]
Radiation exposure in pregnancy is an important topic. While there are prospective studies in humans assessing the actual risk, there is radiation exposure information available from plant leaks and atomic bombs. The average person’s and even physician’s understanding of this topic is poor at best. It is agreed that no exposure to radiation is ideal, however a cumulative exposure of 5 rads is considered the threshold above which an increase in birth defects is seen. The following is a summary of the effects of radiation on the fetus at different stages of development: [54,56,57]
• 0-2 weeks: all or nothing effect
• 2-8 weeks (organogenesis) CNS malformations
• 8-16 weeks: mental retardation
• 2nd and 3rd trimesters: childhood malignancies
It should be noted that there have been no studies showing exposure to 50mGy or less (less than the radiation in most diagnostic tests), causing growth or congenital defects.  Nevertheless a log of all radiation exposure during pregnancy should be kept in the patient’s chart.
Medical management follows the same principals as for the general population, including hydration, analegesics, and antiemetics. The majority of stones, some 70-80%, will pass spontaneously. 
Nephrolithiasis in Children:
Although nephrolithiasis is relatively uncommon in children, it is important to examine as a cause of flank pain due to their high likelihood of recurrence and their possible indication toward underlying metabolic disorder.  The characteristics and treatment of stones in children differ based on stone location and patient age. Pietrow et al describe both intraureteral stones and larger, intrarenal stones in patients of all ages, with intrarenal stones more common in patients less than 5 years of age and the rate of their spontaneous passage in these patients less than 25%. In those patients over the age of 11 years, the rate of spontaneous passage increased to approximately 50%. Many of these cases were associated with furosemide therapy for pulmonary diseases and renal tubular acidosis. In patients 10 years or older, more intrinsic renal illnesses such as hypercalciuria and hypocitruria were found.  Patients with hypercalcuria were more likely to present with flank pain. 
Children older than 5 years old are more likely to present with flank pain. This would correlate with the previous study showing that interureteral stones are more common in older children, as ureteteral obstruction would lead to renal colic. [59,61] Presenting symptoms in the pediatric population with renal colic include flank and abdominal pain in about half the patients, and hematuria in about 90%.  When the stone size was less than 4mm, there was a 66% rate of spontaneous passage in both groups and conservative management was recommended for smaller stones. 
Protease inhibitor stones:
Indinavir, a potent protease inhibitor used for the treatment of HIV, has been associated with nephrolithiasis, with cases even presenting years after discontinuation of therapy.  Twenty percent of the drug is excreted in urine with about half of it unchanged, with the excreted crystals causing stone formation in about 4% of patients on the medication.  Urinalysis may be helpful as the characteristic Indinavir crystals may be visible. While patients present with the typical ureteral colic symptoms, radiologic confirmation of the diagnosis is difficult as these stones are classically described as radiolucent. One study showed 50% of stones were unseen on CT and those seen were likely mixed calcium or uric acid stones. [63,64] In many cases secondary signs of obstruction are used to aid in the diagnosis, such as hydronephrosis or ureteral dilation with columning on IVU. In diagnosed patients, treatment is cessation of the drug, hydration, and analgesics.  Any HIV treated patient presenting with flank pain should be examined for nephrolithiasis.
The laboratory tests obtained in a patient with flank pain are dependent upon the working diagnosis at the time of evaluation. For example, a young male with a history of ureteral colic may need nothing more than a urine dipstick analysis, while a hypotensive middle aged male may need a CBC, type and cross, PT/INR, electrolytes and BUN/Cr.
Few tests are riddled with as much myth and controversy as is the urine sample. Everything from its form of collection, to its time of analysis, to the interpretation of its results is poorly understood by most and many misconceptions abound. Many urinary abnormalities can also be found in disease processes unrelated to the genitourinary system. In a review of 113 patients, Kretchmar and McDonald found that 21% of patients who were diagnosed with appendicitis had pyuria. They also found that patients with an abnormally located appendix had 2.5 times the incidence of pyuria. [65,66]
Starting with the method of collection, the concern with female patients is in obtaining a clean, uncontaminated sample, which is perceived to be challenging. In 1996, Prandoni et al, showed no significant difference in urine culture results- numbers, types, or counts of organisms grown- between clean catch midstream samples and midstream samples. 
Squamous epithelial cells:
Squamous cells are believed to be evidence of contamination in a urine sample. This, however, is not true. Walter et al obtained clean catch midstream urine samples followed by catheterization from 105 women. They showed that 94% of the catheterized specimens contained squamous cells and that they were not an indication of contamination. They noted the epithelium of the vulva, vagina, and urethra as possible sources of squamous cells, as well as squamous metaplasia of the bladder, which has been found in up to 50% of autopsied women and is considered to be a normal variant. 
Urine can be evaluated by dipstick or urinalysis, which must include microscopy. It is evaluated for the presence of blood and evidence of infection. While abnormal microscopic hematuria is defined as the presence of 2 or more RBCs/hpf for asymptomatic patients, there is no consensus on symptomatic patients. [1, 69] Press et al reviewed the results of 109 patients who presented to the emergency department complaining of flank pain and had a urinalysis and IVU to confirm a urolithiasis diagnosis. With urinalysis alone, they reported a sensitivity of 85.5% and a specificity of 42.6% and when combined with dipstick results, 94.5% and 35.2% respectively. They attributed the increased sensitivity with dipstick to a possible loss of RBCs with centrifuging, hemolysis secondary to specimen sitting for a prolonged period of time, and dipstick being performed immediately after obtaining the specimen.  Using CT as the gold standard for diagnosis and greater than one RBC/hpf as a positive result, Bove et al reported a sensitivity of 81% with urinalysis and greater than 1 RBC and 80% for dipstick. 
Overall the sensitivity of hematuria for ureteral colic appears to be between 81%-95.5%. [1,71,72] One study found the sensitivity to be even lower when looking for microhematuria with urolithiasis.  Regardless of the cutoff used, the urinalysis is neither sensitive nor specific with regards to the degree of ureteral obstruction present in a particular patient.  Approximately 15% of patients with ureteral colic will have no hematuria; therefore, overreliance on this test is discouraged. Results can also be altered due to timing of the test. Hematuria has been found to correlate with a particular time period of renal stone. One study found that hematuria becomes less sensitive with time, with sensitivity of 0.95 on day 1, decreasing to 0.68 on day 8. 
Though hematuria is highly sensitive to renal colic, it is not specific (48%).  A study by Jaffe et al found that of those with persistent hematuria, negative cystoscopy and renal ultrasound, 14% were found to have abnormalities with IVU, such as stones or ureteral tumors.  Other causes of hematuria that are relevant to this discussion include urinary tract infection, appendicitis, diverticulitis, leaking abdominal aortic aneurysm, and torsion of ovarian masses. In addition loin pain-hematuria syndrome is also rising in incidence. Though the etiology of the disease remains unknown, it is suggested that the hematuria itself is the cause of flank pain due to irritation and damage to the glomerulus.  The breadth of this differential may further cloud the evaluation of a patient presenting to the emergency department with flank pain. [1,71]
Leukocyte esterase and nitrite:
Leukocyte esterase, part of the urine dipstick (UDP) test, has a sensitivity of 75%-96% and specificity of 94%-98% for greater than 10 WBC/hpf. The nitrite test, designed to identify bacteria, has a sensitivity of 35%-85% and specificity of 92%-100%. A study of 422 men showed the sensitivity of nitrite of 47% and 98% specificity. Combining those results with those of the leukocyte esterase did not improve these values.  Other studies have proven the nitrite test to have a much higher specificity than that of leukocytes esterase.  False negatives can be secondary to low dietary nitrate, use of diuretics, or non-nitrate reductase producing bacteria such as Staphylococcus, Pseudomonas, and Enterococcus.  Lachs et al proposed spectrum bias as a possible cause of the wide-ranging sensitivity and specificity of leukocyte esterase and nitrite. They suggest higher sensitivities and specificities were seen in those with higher pretest probability of UTI. 
Urinalysis versus urine dipstick:
There seems to be no significant difference between UDP and urinalysis. However, both should be interpreted in the clinical context. A negative test in a patient with UTI symptoms should not cause a delay in treatment. [80,81] The utility of the test is small in patients who have either a high or low pre-test probability for UTI; it is highest in patients with moderate probability where it may aid the clinician when evaluating a patient with less than classic symptoms.  Dipstick has some significant advantages such as ease of use, availability, rapidity of results, affordability, and overall respectable sensitivity. 
It is unlikely that a urine culture will significantly alter the ED management of a patient with flank pain as uropathogens are fairly predictable. However, there are cases where obtaining a culture is advisable: recurrent UTI’s, treatment failure, immunocompromised patients, pregnancy, suspected infection with obstruction, recent instrumentation, male patient, or advanced age. 
The proper method of obtaining a urine culture has also been controversial. Lifshitz et al reported no difference in the contamination rates in urine samples obtained from young, outpatient women using three methods of urinating into a clean container: without cleansing, midstream clean catch with spreading of the labia, and the latter in addition to a tampon in an attempt to decrease vaginal contamination.  The study was limited to the use of clean as opposed to sterile containers. Additionally, there has also been shown a 92.3% correlation between cultures obtained from catheterization as compared to ones obtained from the disposable diapers. 
A CBC is usually of limited benefit. An elevated white blood cell count may indicate an infection or demargination caused by pain and stress. [9,50] While 80% of patients with acute appendicitis will have an elevated WBC or neutrophilia, 20% of adults and a higher number of children will present with normal CBC. 
The hemoglobin and hematocrit are important to obtain in a suspected AAA or ruptured ectopic pregnancy and any other situation where acute blood loss or anemia is suspected. Thrombocytopenia is associated with a poor outcome in patients with emphysematous pyelonephritis. [41,44]
Electrolytes and BUN and creatinine are usually obtained as a panel and are commonly normal in most patients. High yield patients may include those with kidney disease, diabetes, intractable vomiting, or prolonged inability to tolerate food. Diabetic patients with a serious infection could be in DKA. If an elevated BUN/Cr is discovered in a patient, it is unlikely to be caused by the acute uretereal obstruction because bilateral obstruction or obstruction of a solitary kidney would have to be present for it to occur. This is usually corrected in the serum due to increased flow to the contralateral kidney in normal patients. 
Elevated LDH is considered a good screening marker for emboli and in one retrospective study of 17 patients there was a 100% correlation with renal artery occlusion.  It is often markedly elevated in these cases. [27,84] However, it is nonspecific, found in different conditions, and there are no good prospective studies validating its use. 
In the context of pyelonephritis, blood cultures would add little if anything to the management of the patient except in cases where a hematogenous source of the pyelonephritis is suspected, such as in endocarditis or intravenous drug abuse. Moreover, in cases of pyelonephritis the blood culture very rarely grows an organism different from that grown in the urine making its significance minimal. [50,85]
PT and INR are indicated in patients with underlying bleeding disorders, liver disease, severe malnutrition, and medication use such as coumadin.
Type and Cross:
Type and Cross should be sent on all patients with suspected AAA, anemia, ectopic pregnancy, or hemodynamic instability.
Urine or serum pregnancy test should be ordered for all female patients of child-bearing age with flank or abdominal pain to rule out suspected ectopic pregnancy, though most cases are usually associated with abdominal pain.  Whether or not a patient is pregnant can also determine appropriate diagnostic tools and treatment. [56,57]
In most cases, a suspected diagnosis will be confirmed or disproved by an imaging study. There is not a single laboratory test that can definitively make the diagnosis. The question then is whether to obtain a study and, if so, which one. It is important to note that making a diagnosis on history and laboratory tests alone will be a “presumed” diagnosis at best. Having said that, the young male who presents with acute flank pain radiating to his groin with hematuria, a history of ureteral colic and no other medical problems may not need a confirmatory test, while a middle aged patient with the same scenario most likely would.
There are a multitude of studies that demonstrate the importance of obtaining confirmatory imaging studies and high rate of 10-30% of alternate diagnoses that are made after noncontrast CT scan. [1,87,88]
When evaluating a patient with flank pain there are a number of important factors to consider in deciding the proper imaging study. First and foremost is ruling out a life-threatening disease. Second is an understanding of ureteral colic and obstruction, and that the mere presence of a stone in the ureter does not necessarily explain symptomatology.
There are a number of imaging studies that may be used to confirm diagnosis and a brief discussion of each follow.
Plain Abdominal Radiographs (PAR):
The use of PAR for the diagnosis of ureteral stones began in 1897 with Swain and became the basis for the assumption that 90% of renal stones are visible on radiograph. [1,89] Multiple studies have subsequently shown the sensitivity of PAR to be between 44%-77% and specificity of 69%-87%. [1,89,90,91,92] PAR is therefore neither sensitive nor specific for the diagnosis of ureteral colic. Also, the mere visualization of a calcification does not explain symptomatology [89,92]. Other causes of calcification seen on PAR include phleboliths, fecaliths, or renal calcifications. PAR is recommended for patients with a history of radioopaque stones and symptoms similar to a previous episode of renal colic, though there is no data indicating the sensitivity or specificity of PAR in this subset of patients.  PAR may miss non-calcium based stones and may not be visible if overlying bone. There are also conflicting studies as to whether PAR better measures stone size as compared to NCCT, which can be important as stone size can direct patient treatment. When comparing NCCT to PAR, one study found that NCCT underestimates stone size, especially those greater than 5mm, while another center found their differences to be negligible. Others have found NCCT to overestimate stone measurement. Many of these studies had small patient populations and multicenter studies could help to reach a consensus. [93,94,95,96]
AAA may be visible on PAR anterior to the spine on a lateral film and left of the midline in a supine anteroposterior projection. However, calcification must be present in both opposing abdominal walls to outline AAA on PAR. A study by LaRoy et al found 55-85% of AAA had enough calcification to be seen on PAR, though follow up studies are required to further the diagnosis. 
Recently noncontrast CT has become the most frequently used tool in the diagnosis of nephrolithiasis. In 1995 Smith et al performed a prospective study comparing noncontrast CT (NCCT) to IVU (the gold standard at the time) in the evaluation of acute flank pain presumed to be ureteral colic. The study was stopped after 22 patients because NCCT was shown to be more accurate than IVU in identifying stones. [87,98,99] Miller et al studied 106 patients with flank pain with NCCT followed shortly by IVU. With CT, they found a sensitivity of 96% and specificity of 100% as compared to 87% and 94%, respectively, for IVU in the detection of stones.  Multiple subsequent studies have confirmed the high sensitivity and specificity of NCCT in the detection of nephroureterolithiasis. [98,100] Even when detecting smaller stones and using larger CT cuts (5mm and greater), there is only a 2-7% false-negative rate. 
All calculi, except for pure Indinavir calculi, are visible on NCCT because of higher attenuation of most stones compared to surrounding tissue.  NCCT can visualize calculi in the bladder that are not otherwise seen on IVU.  Size and location of stones can also be accurately identified and, in the proper setting, it has been suggested that the degree of obstruction can be estimated by the severity of the hydronephrosis.  Again, a broad review of the literature contains numerous cautions to avoid this latter assumption; one may be completely obstructed with no hydronephrosis evident. [1,103] Recently studies have been conducted comparing the densities of the renal parenchyma in those with confirmed ureteral stones. Parenchyma appears thicker in those with an obstructive stone. It may clinically useful to study whether thickening corresponds to degree of obstruction. 
NCCT can be performed quickly, does not require the use of oral or IV contrast agents and does not require bowel preparation.  Perhaps the biggest advantage of NCCT is its ability to provide alternate diagnoses, both urologic and nonurologic. [87,103]. In cases where further investigation is needed, the CT can be repeated with the addition of contrast.
The greatest disadvantage to NCCT is that the patient may be too unstable for transport to the scanner. Other disadvantages of NCCT include inability to assess renal function and misinterpretation of other dense structures such as phleboliths and surgical clips. [99,105] While function cannot be assessed directly, Boridy et al concluded that the degree of perinephric edema seen on CT scan be used to estimate the degree of ureteral obstruction in most patients with renal colic.  This conclusion was based on a retrospective study of 82 patients who had undergone CT followed shortly by IVU. Another study concluded that size, location, and composition of the stone could be used to determine management and that signs of obstruction were not statistically significant between the patients treated conservatively and those who underwent intervention, although this study did not address the emergency management of these patients. 
Radiation exposure is also gaining attention as a disadvantage to NCCT, although the amount of rads reported by different studies vary widely. [107,108,109] The amount of radiation exposure is dependent on multiple factors that are beyond the scope of this article. However, many investigations have been completed and currently are being conducted to reduce the amount of radiation exposure by manipulating various factors. [10,107] Sensitivities of 91%-96% and specificities of 96%-97% have been reported with these modified settings. [107,108,109] Even with decreasing radiation by greater than half, specificity and sensitivity for detecting renal calculi remained 96% and 97% with a 99% positive predictive value. Only those patients with a BMI > 31 would require a higher radiation dose with NCCT.  Another study showed in patients less than 90kg, the accuracy of reduced dose CT was 90% for renal stones and 94% for stones in the ureter, though the population size was small. 
Radiation appears to be of most concern in those patients who would require repeat scans such as those with urolithiasis. In one study of 356 patients who presented to the ED with suspected renal colic, 79% underwent 2 or more CT scans over a 10-month period.  Numerous studies have been conducted to attempt to quantify average radiation exposure, from 8mSv in the average effective dose of a single abdominal CT, 1.18 to 37.66mSv exposure to a patient during a single stone episode, to 29.7mSv median exposure to patients with urolithiasis within a one-year period. [112,113,114] This is especially alarming given that there have been studies showing a significantly elevated cancer risk at 50mSv exposure, and increased risk associated with increasing radiation exposure. Though the recent increase in CT use does not appear to have impacted the rate of cancer in the United States, there is evidence that possible cancer effects of these studies will not be observed until decades after exposure.  A large cohort study by Sodickson et al looked at patients who had recurrent CT scans and predicted their cancer risk. Seven percent of their patients were calculated to have a lifetime attributable risk (LAR) incidence of 1% above the general population, while 3% have a 1% increased risk of cancer mortality.  To put the risk in perspective, one study projected that of all patients in the United States who underwent CT scans in 2007, we can expect 14,000 future cancers due to abdomen and pelvis CT alone.  As patients with renal colic are likely to have repeated exposure to radiation, it would be beneficial for the EP to familiarize themselves with these risks and also consider low dose CT or alternative diagnostic studies.
NCCT is important tool in the diagnosis of flank pain etiology especially in those patients with unclear clinical symptoms. It has become a valid and sensitive modality for the diagnosis of acute appendicitis. A review by Lane and Mindelzun of 256 CTs showed a sensitivity of 95% and specificity of 98%.  A similar study showed the sensitivity and specificity to be high in diagnosing acute appendicitis (96.5% and 98% respectively).  It is also of limited value in very thin patients who do not have enough intra-abdominal fat to provide adequate contrast. 
NCCT is also the study of choice to detect emphysematous pyelonephritis. While contrast CT may also be used for detailing gas characteristics, NCCT has proven to be useful in the disease diagnosis. 
In AAA, CT scan provides good evidence of location, size, rupture, and extent of hemorrhage. It has a sensitivity and specificity of 90-94% and 92-100%, respectively. [5,18] Again, the patient’s stability for transport to the scanner must be addressed.
While enhanced CT is well established as a valid modality in detecting renal artery occlusion, after an extensive search of the literature it is unclear if NCCT has any role in diagnosis. More importantly, it is unclear if there are signs that would be apparent on NCCT that would prompt the physician to further evaluate using contrast. Therefore, until more information is available, the EP will likely have to follow a NCCT with an enhanced study and hope that the initial evaluation did not lead them in the wrong direction.
The cost of NCCT is institution dependent. As NCCT becomes more common in the evaluation of flank pain, some institutions may lower cost for this purpose.
Beginning with its first use in 1923, IVU became the gold standard for the diagnosis of renal colic.  This test has the advantage of utilizing technologies and personnel already present in the hospital and may be performed and interpreted readily by the EP. The basic procedure involves the injection of contrast dye followed by plain radiographs of the kidney, ureters, and bladder in an attempt to determine the level, cause, and severity of the obstruction. The sensitivity of IVU is approximately 90% and its chief advantage is its ability to provide gross estimate of renal function . Other information readily available from an IVU includes an estimate of the degree of hydronephrosis, the relationship of suspicious calcification to the collecting system, and the anatomy of the ureteral system. [88,98,120]
IVU has since been replaced by NCCT has the imaging of choice for diagnosis of renal colic. The main disadvantages of IVU are related to the use of contrast and the duration of time needed to complete an adequate study. The contrast is normally iodinated material and in 5-10% of the general population is associated with adverse systemic reactions ranging from nausea and vomiting to urticaria, bronchospasm, arrhythmia, nephrotoxicity, and anaphylactoid reactions. [1,119] It is contraindicated in any patient with a known allergy to contrast and relatively contraindicated in patients with: renal insufficiency (CR>1.8), diabetes mellitus, multiple myeloma, and dehydration.  The incidence of nephrotoxicity in predisposed patients can be as high as 25%.  In patients receiving ionic high osmolar contrast, the rate of non-life-threatening adverse reaction is 1-2%, while life-threatening reactions occur in about 0.2% of patients. These rates are lower in patients receiving low osmolar non-ionic contrast: 0.2-0.4% for non-life threatening reactions and 0.04% for life threatening reactions. The mortality rate is around 1 in 75,000 regardless of type of contrast used. In diabetics taking metformin, there is a risk of developing lactic acidosis. This is due to the drug accumulation rather than a reaction with the contrast medium. In the US, the current recommendation is to stop metformin at the time of the study and for 48 hours afterward. 
The time course needed to complete an IVU in the emergency department can be lengthy particularly taking into account the time needed to verify a normal creatinine. It can be prolonged further if there is an obstruction and delayed films are needed to properly opacify the collecting system.  Like PAR, while the IVU is unable to demonstrate a radiolucent stone directly, the surrounding dye will allow a negative image of the obstruction, or “filling defect” to be visualized. This will allow the evaluation of ureteral or nephretic dilation and provide information as to the level of any obstruction. 
The radiation dose of IVU depends on the number of films obtained. This dose is approximately 1.5 mSV for a three-shot study. 
Yet another modality that can be used in the patient with acute flank pain is ultrasound. It is readily available, noninvasive, independent of stone composition, and does not involve radiation. [1, 105] It is the study of choice in pregnancy and in children. Stones that are directly visualized are usually in the intrarenal collecting system or near the ureterovesical junction. Small stones (especially those less than 2mm) or ureteral stones are seldom visualized directly. [86,105] Children are an exception to this due to their generally small bodies. 
Important are secondary signs of obstruction, which include hydronephrosis, a dilated collecting system, and/or a dilated proximal ureter. However, these are unreliable signs in early, partial, or intermittent obstruction as hydronephrosis may be minimal.  Moreover, patients with renal colic may be dehydrated from repeated vomiting which may decrease the sensitivity of ultrasound detection of a stone to about 35% [56, 57].
Ultrasound is dependent on multiple factors beginning with operator skill and experience. Additionally, patient body habitus, intestinal gas, and the use of NSAIDS prior to exam can significantly decrease the yield . Another limitation of ultrasound is its inability to elucidate stone size, an important factor in patient management. 
Reports regarding the sensitivity and specificity of ultrasound are variable and may be due to the limitations stated above as well as the hydration status of the patient. Ultrasound has been found to have a sensitivity of 86.8% for hydronephrosis.  Sinclair et al reported sensitivity of 85% when hydronephrosis or hydroureter were considered positive results and 64% when actual visualization was considered positive.  There have been studies by Haddad et al, and Kartal et al, which report sensitivity of 81-92% for detection of pelvicaliceal dilatation. [124,126] Ultrasound has been shown to have a 90% sensitivity in the direct detection of larger stones. 
Improved visualization of the distal ureter may be achieved by transvaginal and transrectal ultrasound. Laing et al proposed using transrectal ultrasound in males and vaginally in females. [127,128] However, the significant discomfort and invasiveness of these modalities limits their use.
Doppler ultrasound has also been used in the evaluation of renal colic. The resistive index (RI) and the difference in RI (dRI) are measured in Doppler ultrasound. RI is defined as (peak systolic velocity - the end diastolic velocity)/ peak systolic velocity. The dRI is the difference in RI of the corresponding and contralateral kidney. The determination of the RI is based upon the observation that ureteral obstruction consistently produces a transient renal vasodilation mediated by local prostaglandins followed by vasoconstriction.  Shokeir et al prospectively studied 117 patients and found a sensitivity of 77% and specificity of 83% for RI and 88% and 98% respectively for dRI.  Reported sensitivities of RI range between 57% and 100%. [129,130,131] Doppler ultrasound has multiple limitations:
• It is less sensitive in partial ureteral obstruction.
• NSAIDs can reverse the early vasodilation and later vasoconstriction that occur in acute renal colic 
• Bilateral obstruction and obstruction of a solitary kidney can confuse the operator.
• In early and intermittent obstruction, intrarenal pressure may still be in the normal range, resulting in a “normal” study.
• The dRI is chronically elevated in hypertension and renal disease and may lead to false positive results. 
Ureteral jets into the bladder have also been studied in the setting of ureteral obstruction. For this technique to be successful, the patient must be adequately hydrated, with a partially emptied bladder to provide adequate background contrast. Both ureteral orifices are visualized simultaneously to differentiate between the jets originating from each side. Ureteral jets have not been found to be very sensitive with one study reporting 30% of patients with obstruction having normal jets. Limitations of this method include the length of time needed for a proper study (5-20 minutes), partial obstruction, or inability to adequately hydrate patients with underlying cardiac or renal disease. 
In a small study with a convenience sample of 68 patients, Kuhn et al evaluated the usefulness of bedside ultrasound performed by EP’s in ruling out an AAA. Sensitivity and specificity was reported at 100%. It was concluded that bedside ultrasound can effectively used by EP’s without extensive training in ultrasonography and would aide in the rapid identification of AAA and those in need of emergent surgery. However this was a small study with potential bias and other flaws.  Another study by Shuman et al found a 98% sensitivity for detecting AAAs but only 4% sensitivity for detecting extraluminal blood in a prospective study of 60 patients. Unlike the above study, they had a sonographer performing the ultrasounds rather than EPs.  The limitations with US in detecting AAA include patient body habitus, overlying gas and poor visualization of renal arteries. 
Ultrasound may be used to detect emphysematous pyelonephritis. Intraparenchymal gas is visible on ultrasound. However, in the presence of diffuse gas in the perinephric space, the image will manifest as a “gassed-out” kidney and not be visible. 
Combined ultrasound and plain film:
This combination was proposed as a way of compensating for the difficulty of detecting stones by ultrasound outside of the UPJ or UVJ. A prospective study by Ripolles et al found a 64% sensitivity of plain film in detecting stones, which may contribute the 100% sensitivity of ultrasound.  This study suggests the sonographer use plain film as a guide to the possible site of obstruction. A subsequent prospective study by Palma et al showed a sensitivity of 95% and specificity of 67% for the combination using radiologists as the ultrasonographers and IVU as the gold standard.  Henderson et al in 1998 performed a similar prospective study using 2nd, 3rd, and 4th year emergency medicine residents as the ultrasonographers. Similar high sensitivity and low specificity, 97.1% and 58.9%, were found.  This combination of sensitivity and specificity in both studies was due to a substantial number of false positives.
Once a diagnosis of renal colic is made, then the EP must evaluate for presence of infection and obstruction, which would require emergent urologic consultation, admission, IV antibiotics, and surgical drainage. Pain or vomiting that is intractable to ED management requires admission and urologic consultation as well. As renal colic is the most common cause of flank pain, the following will detail the management of renal colic for the EP.
Stone size and rate of passage:
Multiple studies have shown that stones that are less than 5mm usually pass spontaneously, however, reported rates vary in the literature. Coll et al used CT scanning to determine the relationship between stone size, location, and spontaneous passage. Their findings are as follows: 
• 78% for stones <4mm
• 60% for stones 5-7mm
• 39% for stones >8mm
• No stones greater than 10mm passed spontaneously
Location of the stone was also assessed with regards to rate of spontaneous passage: 
• 50% for proximal ureter stones (above sacroiliac joint)
• 60% for mid ureter stones (overlying sacroiliac joint)
• 75% for distal ureter stones (below sacroiliac joint)
• 79% for stones in the ureterovesical junction
A similar trend was seen with combined size and location and rate of spontaneous passage; ie, small distal stones had a higher rate of passage than larger proximal ones. 
Current guidelines suggest that ureteral stones <10mm with symptom management may be observed and given medication to allow for passage of the stone. 5mm stones have been reported to have a 68% passage rate while those 5-10mm have a 47% passage rate. Those stones in the proximal ureter are less likely to pass spontaneously. 
Even in management of pregnant women diagnosed with renal colic, 50-80% only require hydration and analgesics to pass the stone spontaneously. 
Stone size and time of passage:
Miller et al studied the time to stone passage and concluded that the interval to passage was variable and dependent on the stone size and location. In general, proximal obstructing stones tend to be larger than those more distal, though smaller distal stones passed sooner than large proximal ones. The days to stone passage were 8.2 days for stones 2mm or smaller, approximately 12 days for stones measuring 3mm, and 22 days for those 4-6mm. [137,138,139]
In 1979 Edna and Hasselberg randomized 60 patients with proven ureteral colic to receive either 3 liters of fluids or none. They found no difference in the pain level or the likelihood of spontaneous passage in the two groups. The fluid given doesn’t “flush” the stone out of the ureter. In fact studies indicate that most of the fluids administered are filtered through the contralateral kidney.  It seems the benefit in hydration is more for the prevention of stone recurrence. One prospective study reported a recurrence interval of 38.7 +/- 13.2 months in the hydrated group versus 25.1 +/- 16.4 months in the non-hydrated group. 
Pain in renal colic is likely due to dilation of the ureter secondary to distal obstruction.  Ureteric obstruction causes the kidney to produce and release prostaglandins that mediate renal vasodilation and increased blood flow in addition to increasing ureteric smooth muscle contraction. [143,144] Nonsteroidal anti-inflammatory medications inhibit prostaglandins and, thereby, decrease renal blood flow and diuresis. They also decrease ureteral smooth muscle contraction, reduce inflammation, and decrease capsular distention and associated pain. [144,145]
In the United States, ketorolac is the only NSAID approved for parenteral use. Larkin et al compared the efficacy of intramuscular ketorolac versus meperidine for the treatment of patients with renal colic. They found that ketorolac was more effective than meperidine in relieving pain and patients who received the former were discharged home earlier (52 minutes average) than those receiving meperidine. However, they noted that the delayed discharge in the meperidine group was likely due to multiple factors including increased sedation, perception on behalf of staff that patients receiving meperidine required prolonged observation, and/or improved resolution of pain with ketorolac.  Other studies have shown similar efficacy although there were problems with the medication doses; typically too much of ketorolac and too little meperidine. [143,144] A subsequent study by Holdgate and Pollack found that those treated with NSAIDs versus opoids reported less use of rescue medication and less adverse reactions such as vomiting. 
NSAIDs are contraindicated in patients with borderline renal function, peptic ulcer disease, known hypersensitivity, and pregnancy due to the theoretical risk of premature closure of the ductus arteriosus when given near-term and an association with fetal pulmonary hypertension.  Pain treatment then for pregnant patients with renal colic allows them to be given opioids such as meperidine and morphine, which are safe in pregnancy.
Recent studies however have shown that stone passage can be aided with addition of alpha-blockers or calcium channel blockers. Hollingsworth et al reported that those patients give tamsulosin or nifidepine had a 65% greater chance of stone passage while another study found tamulosin to increase rate of stone explusion in the distal ureter. [148,149] Seitz et al found that medical therapy not only increases passage of stones, but also can decreases time of passage, and lowers analgesia requirement, however this study was combined with extracorporeal shock wave lithotripsy. 
Most stones should pass within 4-6 weeks. A suggested algorithm for confirmed renal colic must first rule out emergent disease, such as urosepsis or problems with urine production, in which case the patient should be immediately referred to a urologist. Patients should be admitted if their pain or other symptoms are intractable or the patient presents with multiple other comorbid conditions.  Based on current guidelines the EP should advise the patient presenting with an initial case of lithiasis <10mm on medical therapy to obtain follow up studies to assess for stone movement and to monitor for signs of hydronephrosis. If the stone fails to progress, colic symptoms do not improve or there is evidence of obstruction then patient will be referred to a urologist for stone removal. Children should be referred to a urologist and due to the small size of their ureter and urethra treatment tends to favor shock-wave lithotripsy. 
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