Urinary ascites results in pseudoazotemia due to urinary creatinine reabsorption across the peritoneum. We report a case of a pyeloplasty complicated by urine extravasation, in which the diagnosis was aided by discrepant findings of an elevated serum creatinine level but a stable cystatin C level. Cystatin C is a marker of renal function but is not typically excreted into the urine and therefore can be used to differentiate pseudoazotemia from true azotemia and is a better marker of renal function in the setting of known urinary ascites. These findings are relevant for patients with potential traumatic or nontraumatic sources of urine extravasation.
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