eMedicine Specialties > Emergency Medicine > Genitourinary

Renal Calculi: Differential Diagnoses & Workup

Author: Sandy Craig, MD, Adjunct Associate Professor, Department of Emergency Medicine, University of North Carolina at Chapel Hill, Carolinas Medical Center
Contributor Information and Disclosures

Updated: Oct 29, 2009

Differential Diagnoses

Other Problems to Be Considered

Pyonephrosis
Renal artery embolus

Workup

Laboratory Studies

• Urinalysis
  • One retrospective study found that 67% of patients with ureterolithiasis had more than 5 RBC per high power field (hpf) and 89% of patients had more than 0 RBC/hpf on urine microscopic examination.⁷ In addition, 94.5% have hematuria if screened with microscopy plus urine dipstick testing.⁸
  • Degree of hematuria is not predictive of stone size or likelihood of passage.
  • No literature exists to support the theory that ureterolithiasis without hematuria is indicative of complete ureteral obstruction.
  • Pyuria (>5 WBC/hpf on a centrifuged specimen) in a patient with ureterolithiasis should prompt a careful search for signs of infected hydronephrosis. Obtain a complete blood count (CBC), creatinine, and urine culture. Treatment with antibiotics is indicated in patients with ureterolithiasis and pyuria. Admission to the hospital is mandatory if the patient has any signs of infected hydronephrosis (fever, elevated WBC count, elevated creatinine), or if follow-up within 24 hours is not reliably available.
  • A urine pH greater than 7 suggests presence of urea-splitting organisms, such as Proteus, Pseudomonas, or Klebsiella species, and struvite stones.
  • A urine pH less than 5 suggests uric acid stones.
• Electrolytes
  • Serum creatinine level is the major predictor of contrast-induced nephrotoxicity.
  • If creatinine level is greater than 2 mg/dL, use diagnostic techniques that do not require an infusion of contrast, such as ultrasonography or helical CT scan.
  • Hypokalemia and decreased serum bicarbonate level suggest underlying distal (type 1) renal tubular acidosis, which is associated with formation of calcium phosphate stones.

Imaging Studies

• Most authors recommend diagnostic imaging to confirm the diagnosis in first-time episodes of ureterolithiasis, when the diagnosis is unclear, or if associated proximal UTI is suspected. Lindqvist et al found that patients who are pain-free after receiving analgesics could be discharged from the ED and undergo radiologic imaging after 2-3 weeks without increasing morbidity.⁹
• Kidney, ureter, and bladder (KUB) radiography
  • Multiple studies show that the KUB radiography has low (40-50%) sensitivity and specificity for the presence of ureterolithiasis and adds nothing to the emergent clinical impression. At follow-up, the urologist may find the KUB radiograph to be helpful in determining the exact size and shape of the stone, in establishing a baseline for follow-up studies, and for visualization of the surgical orientation.
  • KUB radiographs can be used to monitor passage of a previously documented opaque stone. Note that most stones will appear larger on KUB radiograph than on CT, with CT-based measurement of maximum stone dimension approximately 12% smaller compared with a KUB-based measurement.¹⁰
• Computed tomography (CT): Noncontrast helical CT has become the criterion standard imaging study in the ED diagnosis of ureterolithiasis (see Media file 1).

• Numerous studies have demonstrated that CT has a sensitivity of 95-100% and superior specificity and accuracy compared with the historic criterion standard, intravenous pyelogram (IVP).¹¹
• Other advantages of helical CT include rapid (<5 min) acquisition time, avoidance of intravenous (IV) contrast, and potential for diagnosis of other pathology including AAA, pancreatitis, appendicitis, ovarian disorders, diverticular disease, renal carcinoma, and biliary tract disorders.¹²
• Principal disadvantages are that helical CT gives no information on renal function or degree of urinary obstruction. A recent study also demonstrated that stone size as measured on CT KUB correlates poorly with actual size of the stone measured after spontaneous passage.¹³ For this reason, caution should be used in counseling patients on the likelihood of spontaneous stone passage when stone size is determined using CT-based measurement.
• Pure indinavir stones are radiolucent and may not be visualized well by helical CT scan. However, indinavir stones often serve as a nidus for deposition of calcium oxalate or calcium phosphate deposition and thus become radioopaque. CT KUB remains the test of choice for patients on indinavir who present with apparent renal colic.
• Sulfadiazine stones, most often seen in AIDS patients taking sulfadiazine for treatment of toxoplasmosis, are also difficult to visualize on CT because of relatively low attenuation.¹⁴
• Intravenous pyelogram: Prior to the advent of helical CT, IVP was the test of choice in diagnosing ureterolithiasis. IVP is widely available and fairly inexpensive but less sensitive than noncontrast helical CT.
  • Contrast is administered intravenously at a dose of 1 mL/kg, and KUB films are taken immediately and at 1, 5, 10, and 15 minutes until contrast fills both distal ureters (see Media file 2). Look for direct visualization of stone within the ureter, unilateral ureteral dilation, delayed appearance of the nephrogram phase, lack of normal peristalsis pattern of the ureter, or perirenal contrast extravasation. Degree of obstruction is graded based on delay in appearance of the nephrogram.

• Anaphylaxis to ionic contrast agents (eg, Renografin, Conray) occurs in 1-2 patients per 1000 IVP studies. Risk of recurrence is approximately 15% if reexposed to ionic agents but falls to 5% when nonionic agents are used. Risk of anaphylaxis can be reduced further by pretreatment with a combination of H1- and H2-blockers and steroids, but studies showing the benefit of pretreatment began pretreatment more than 12 hours prior to study. Risk of nephrotoxicity is not clearly reduced with use of nonionic agents. Indications for use of nonionic contrast agents vary among institutions but consistently include history of prior mild to moderately severe reaction to ionic contrast, asthma, multiple allergies, or severe cardiac disease.
• Disadvantages of IVP include radiation exposure and risk of nephrotoxicity or anaphylactoid reaction to contrast agent. IVP is relatively contraindicated in pregnant or dehydrated patients or if serum creatinine level exceeds 2 mg/dL. IVP is absolutely contraindicated in patients with a history of severe contrast-induced anaphylaxis. False-negative results usually occur with stones located at the ureterovesical junction.
• Ultrasonography: This is a good imaging modality in patients who are pregnant or to rule out the presence of an AAA in patients older than 60 years with a first or atypical presentation of nephrolithiasis.¹⁵
  • A handful of small studies have found sensitivities of 65-100% (see Media files 3-4). Ultrasonography has been found to be less accurate in diagnosis of ureteral stones when compared with IVP or helical CT. Diagnostic criteria include direct visualization of the stone, hydroureter more than 6 mm in diameter, and perirenal urinoma suggesting calyceal rupture.¹¹

• Advantages include lack of radiation exposure and ability to complete the study at the bedside in patients who are potentially in unstable condition.
• Disadvantages include inferior sensitivity, lack of universal availability, dependence on operator expertise, and inability to accurately estimate the degree of urinary obstruction.
• A urine-filled bladder provides an excellent acoustic window for ultrasound imaging; sonograms occasionally may demonstrate a stone at the ureterovesical junction that is not seen on helical CT or IVP.
• Future studies may utilize 2-dimensional ultrasonography in combination with color Doppler analysis of the ureteral jets to enhance sensitivity of ultrasonography in patients with ureteral colic.¹⁶
• Magnetic resonance imaging: MRI can be used to detect ureteral stones. One study of 40 consecutive patients with acute flank pain found sensitivity of 54-58% and specificity of 100% using breath-hold heavily T2-weighted sequences.¹⁷ Sensitivity and specificity increased to 96.2-100% and 100%, respectively, using gadolinium-enhanced 3-D FLASH MR urography. Although MRI does not play a major role in the diagnosis of ureteral stones, lack of radiation makes MRI a good choice in pregnant women who have nondiagnostic findings from a sonogram.

Procedures

• Occasionally, a patient may require urinary catheterization to relieve retention due to extreme pain or an obstructing bladder neck stone.

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