The diagnosis of urologic stone disease is clinically suspected and supported by the presence of hematuria; imaging confirms the diagnosis with certainty.
Many diagnoses can be confused with renal colic (Table 94-3). History and physical examination can be difficult because the patient's discomfort may interfere with adequate information collection. The most critical diagnoses to consider are aortic dissection and ruptured abdominal aortic aneurysm. Renal colic and abdominal aortic aneurysm may have similar presentations.
TABLE 94-3Differential Diagnoses for Ureterolithiasis ||Download (.pdf) TABLE 94-3 Differential Diagnoses for Ureterolithiasis
|Vascular || |
Abdominal aortic aneurysm
Renal artery embolism
Renal vein thrombosis
|Renal || |
Renal cell carcinoma
|Ureter || |
Tumor (primary or metastatic)
|Bladder || |
|GI || |
Perforated peptic ulcer disease
|Gynecologic || |
Pelvic inflammatory disease/tubo-ovarian abscess
|GU || |
|Other || |
The laboratory evaluation centers on evaluating for infection, kidney dysfunction, and possibility of pregnancy. Test all females of childbearing potential for pregnancy when considering renal colic. With pregnancy, consider ectopic pregnancy in the differential diagnosis while minimizing radiation exposure to the fetus.
Urinalysis is needed to rule out infection. If infection is found, obtain urine culture and sensitivities to guide antibiotic therapy.14 In suspected pediatric nephrolithiasis, a culture is often sent regardless, because identification of a urinary tract infection in children can be more difficult (see chapter 132, "Urinary Tract Infection in Infants and Children").
Hematuria (three or more red blood cells per high-power field), or even its absence, can mislead the physician. Although 10% to 15% of patients with nephrolithiasis will have no hematuria, approximately 24% of patients with flank pain and hematuria have no radiographic evidence of ureterolithiasis.15 Therefore, although hematuria may contribute to diagnostic decision making, it should not be used alone to exclude or confirm the diagnosis of ureterolithiasis15 (see chapter 91, "Urinary Tract Infections and Hematuria").
Check renal function because the overwhelming majority of patients who form stones have reduced creatinine clearance.16 Unless febrile or systemically ill, a WBC count does not aid in the evaluation; many patients will have an elevated WBC count due to stress demargination. Other laboratory studies, such as serum calcium or uric acid, are not useful in the initial evaluation or treatment but help determine stone type and long-term therapy.
Imaging confirms the presence of a ureteral stone, rules out other diagnoses, identifies complications, defines stone location, and assists with management if the stone fails to pass spontaneously.17 Imaging is recommended by the American Urological Association in patients with suspected first-time stones.18 For young, healthy, stable patients with a history of kidney stones in whom the diagnosis is clinically clear, imaging may be deferred until the follow-up visit provided that a reliable follow-up mechanism exists.19 However, clinicians are frequently incorrect in their clinical impression in 20% to 70% of cases.20 CT scanning reveals an alternative diagnosis in 33% of the patients.20 Thus, the physician should determine the need for confirmation of the diagnosis based on the patient's past medical history, dangers of accumulated radiation exposure, clarity of the clinical diagnosis, and ease of follow-up and ability to return for worsening symptoms.
The noncontrast helical CT scan is sensitive and specific, with "diagnostic" positive and negative likelihood ratios for detection of renal stones (Table 94-4).21 Images are obtained from the top of the kidney to the bladder base. Secondary signs of ureteral obstruction, such as ureteral dilatation, stranding of perinephric fat, dilatation of the collecting system, and renal enlargement, can be helpful in making the diagnosis. In combination, unilateral ureteral dilatation and perinephric stranding have a positive predictive value of 96% for stone disease.22 If both are absent, the negative predictive value is 93% to 97%22 (Figures 94-1 and 94-2).
TABLE 94-4Ancillary Tests in Urologic Stone Disease ||Download (.pdf) TABLE 94-4 Ancillary Tests in Urologic Stone Disease
|Test ||Sensitivity (%) ||Specificity (%) ||LR+ ||LR– ||Comments |
|Noncontrast CT ||94–97 ||96–99 ||24–∞ ||0.02–0.04 || |
Advantages: speed, no RCM, detects other diagnoses
Disadvantages: radiation, no evaluation of renal function
|IV urogram ||64–90 ||94–100 ||15–∞ ||0.11–0.15 || |
Advantage: evaluates renal function
Disadvantage: RCM (allergy, nephrotoxicity, metformin related acidosis)
|US ||63–85 ||79–100 ||10–∞ ||0.10–0.34 || |
Advantages: pregnancy, no RCM, no radiation, no known side effects
Disadvantages: insensitive in middle third of the ureter, may miss smaller stones (<5 mm)
|Plain abdominal radiograph ||29–58 ||69–74 ||1.9–2.0 ||0.58–0.64 || |
Advantage: may be used to follow stones
Disadvantage: poor sensitivity and specificity
A. Red arrow shows 6-mm stone within the proximal third of the left ureter on noncontrast CT reformatted image of upright abdomen. B. From same patient as in A, note 6-mm stone (arrow) within the proximal third of the left ureter on noncontrast CT.
CT image shows 5-mm stone (arrow) at left ureterovesical junction. Other calcifications are seen in the pelvis, unrelated to the urinary outflow system.
Noncontrast helical CT has advantages over other imaging modalities, including superior speed, the avoidance of radiocontrast media, and greater ability to identify other pathologies. However, because radiocontrast is not used, the specificity and sensitivity for other diagnoses (e.g., abdominal aortic aneurysm, appendicitis, renal infarct, or perinephric abscess) are not as great as with imaging protocols using contrast, and renal function is not assessed.
Low-dose CT has been studied in small numbers. Low-dose CT is as sensitive as standard CT in detecting stones >3 mm in patients with a body mass index <30. However, it was not as sensitive for smaller stones or at higher body mass indices.23
IV urography (or IV pyelogram) is rarely used, but yields information on renal function and anatomy. It detects calculi with modest sensitivity but excellent specificity21 (Table 94-4). IV urography can be an adjunct to CT if functional information and knowledge of the degree of obstruction are required.
Plain Abdominal Radiographs
Approximately 90% of urinary calculi are radiopaque because calcium phosphate and calcium oxalate stones have a density similar to that of bone. Magnesium-ammonium-phosphate (struvite) is slightly less radiodense, followed by cystine, which is only partly radiopaque. Uric acid and matrix stones are essentially radiolucent, as are most stones associated with medications such as indinavir. Unfortunately, because of small size and overlapping soft tissue and bone shadows, urinary stones are visible much less frequently on plain films. A plain kidney-ureter-bladder film is neither sensitive nor specific enough to rule in or rule out stone. However, once the location of a stone is identified on CT scan, the progression of the stone can be followed by a kidney-ureter-bladder film assuming the stone is visible.
If patients are not candidates for CT due to concerns about radiation (e.g., during pregnancy or in children), US can assist in the diagnosis. Although useful in the detection of larger stones (Figure 94-3), US may miss smaller (<5 mm in diameter) ureteral stones.24 US is helpful in diagnosing stones in the proximal and distal ureters but is insensitive for mid-ureteral stones. Overall, US has only modest sensitivity and specificity for detecting renal stones (Table 94-4) but is 78% sensitive for detecting hydronephrosis. This sensitivity for hydronephrosis goes up from 75% for stones <6 mm to 90% for stones >6 mm. However, of hydronephrosis diagnosed by US, up to 22% of studies do not represent obstruction; but rather, normal anatomic variation, full bladder, and renal cysts. Rapid bolus infusion of crystalloid can result in a false-positive finding of hydroureter.
US of renal pelvis showing stones (marked with 1+ and 2+) with shadowing effects (arrows).
US provides information on renal size and, with Doppler scanning, renal blood and urine flow. Obesity may interfere with obtaining good-quality scans, and US can miss early obstructive signs. Accuracy of the US study is dependent on the skill and experience of the operator.
Although not quite as accurate as a screening examination would need to be, compared with CT, bedside US by ED physicians had a sensitivity of 80% (95% confidence interval [CI], 65% to 89%), specificity of 83% (95% CI, 61% to 94%), and overall accuracy of 81% (95% CI, 69% to 89%) in the detection of hydronephrosis.25,26,27 Bedside US may be useful to detect larger stones unlikely to respond to conservative measures.