Dynamic Discordance Between Creatinine- and Cystatin C-Based Estimated Glomerular Filtration Rates Across Forniceal Rupture and Its Surgical Decompression: A Case Report.

Forniceal rupture from an obstructing ureteral stone can raise serum creatinine, but whether that rise reflects a true fall in filtration or pseudoazotemia from reabsorbed extravasated creatinine is easily missed. We describe a woman in her 70s with calculous forniceal rupture and retroperitoneal urine leakage in whom serial creatinine and cystatin C, together with the difference between their estimated glomerular filtration rates (ΔeGFR, cystatin C-based minus creatinine-based), were tracked over the course of the illness. The gap was large and positive at presentation (+41.7 mL/min/1.73 m²), reversed to a negative nadir on the operative morning (-7.4), when a creatinine that had fallen only to the low-normal range might, on its own, have been mistaken for a recovery that the lower cystatin C-based estimate did not support. It then returned through postoperative day 1 (-2.7) to a modest positive value by day 8 (+7.6), well below the presenting gap though not resolved to zero. Over time, the direction of this discordance helped separate reabsorptive pseudoazotemia from a within-range creatinine that alone would have overstated recovery.