ACCURACY OF HEMATURIA IN DIAGNOSING KIDNEY STONES.
Clinical question Is hematuria predictive of kidney stones in patients with acute flank pain?
Background Previous studies suggest that hematuria is 86% to 100% sensitive for detecting ureterolithiasis in patients with acute flank pain. Although these studies have used excretory urography (IVP) as the reference standard, noncontrast helical computed tomography (CT) appears to be a superior reference standard.[1,2] The authors of this study examined the accuracy of hematuria testing compared with a reference standard of helical CT and attempted to differentiate between dipstick screening and different thresholds of microscopic hematuria.
Population studied The investigators examined the records of 267 patients with acute flank pain referred from an urban emergency department for unenhanced helical CT. Seventy-two patients with no available hematuria testing were excluded. No demographic data were given.
Study design and validity This was a retrospectire study: The researchers did not prospectively enroll everyone with acute flank pain and perform both hematuria testing and CT imaging. Because some patients with acute flank pain were not referred for CT, this introduces a spectrum bias. For example, classic cases may not have been scanned, because the diagnosis was not in doubt. Further, the study was not blinded, no tests for statistical significance were performed, and the authors did not describe specific helical CT diagnostic criteria for ureterolithiasis.
Outcomes measured The primary results were the sensitivity, specificity, positive predictive value, and negative predictive value of hematuria, using a helical CT diagnosis of ureterolithiasis as the reference standard.
Results The prevalence of kidney stones in the study population was 49%. This was similar to the prevalence among the 72 patients without hematuria testing. The sensitivity and specificity for a patient with any degree of microscopic hematuria were 89% and 29%, corresponding to a positive likelihood ratio (LR+) of 1.3 and a negative likelihood ratio (LR-) of 0.4. At a prevalence of 49%, 54% of patients with any degree of microscopic hematuria had a stone, while 74% of patients without any microscopic hematuria did not have a stone. Thus, 26% of patients with flank pain but no microscopic hematuria had a stone by helical CT. Increasing the diagnostic threshold to [is greater than] 1 red blood cell (RBC) per high-power field improves the overall accuracy of the test but does not improve the usefulness of these posttest probabilities very much. For [is greater than] 5 RBCs per high-power field, the sensitivity lowers to 67%, and the specificity improves to 66% (LR+ = 1.97; LR- = 0.5). However, 32% of the sample without this degree of hematuria had a stone on CT. Dipstick screening was even less accurate than microscopic testing.
Recommendations for clinical practice Although this study has numerous methodologic flaws, it does suggest that hematuria does not satisfactorily rule in or rule out kidney stones for patients with acute flank pain. It still may have some diagnostic benefit if the pretest probability is high, such as for patients with known stone disease and typical recurrent symptoms. Given the existing data on hematuria, IVP, and CT, clinicians seeking a definitive diagnosis of ureterolithiasis should consider noncontrast helical CT. It is much more accurate than hematuria testing and is also cheaper, safer, easier, and more accurate than IVP.[1,2]
Clint J. Koenig, MD Erik J. Lindbloom, MD, MSPH University of Missouri Columbia E-mail: KoenigC@health.missouri.edu
[1.] Chen MY, Zagoria RJ. Can noncontrast helical computed tomography replace intravenous urography for evaluation of patients with acute urinary tract colic? J Emerg Med 1999; 17:299:303.
[2.] Niall O, Russell J, MacGregor R, Duncan H, Mullins J. A comparison of noncontrast computed tomography with excretory urography in the assessment of acute flank pain. J Urol 1999; 161:534-7.
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|Author:||Koenig, Clint J.|
|Publication:||Journal of Family Practice|
|Date:||Nov 1, 1999|
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