Pale Kidney Sign- A Useful Ancillary Sign for Obstructive Ureterolithiasis?

Abstract

BACKGROUND This study was performed to investigate the sensitivity of reduced attenuation of unilateral kidney, dubbed the 'pale kidney sign', in identification of obstructive urolithiasis. METHODS In this observational study, 25 patients with symptoms of acute renal colic with unilateral ureteric calculus on CT were included. Attenuation value of each kidney was averaged at 3 poles and attenuation difference between the 2 kidneys was calculated. Cut off of 5 HU (Hounsfield Units) attenuation difference in the obstructed kidney denoted pale kidney sign. Sensitivity was determined using contingency table analysis. RESULTS Among the 25 patients with ureteric calculus, the mean attenuation in obstructed kidney was 27.9 HU (SD 2.7) and mean attenuation in non-obstructed kidney was 33.3 HU (SD 1.9). The difference in attenuation of the obstructed and nonobstructed kidney was on an average 5.3 HU (SD 2.6), with only 2 obstructed kidneys having higher attenuation than contralateral kidneys. Considering the cutoff of 5 HU, 17 of 25 patients had pale kidney sign with sensitivity of pale kidney sign for diagnosis of obstructing ureterolithiasis being 68% (95% CI-46.5% to 88.0%). CONCLUSIONS The pale kidney sign is a useful ancillary sign of obstructive ureterolithiasis with good sensitivity which can potentially be helpful in difficult cases such as those due to technical factors, confusing phlebolith, passed calculus, etc. This can reduce the number of repeat scans and avoid additional patient workup.