Discrepancy between cystatin C and creatinine points leading to a diagnosis of postrenal acute kidney injury and its reversibility: three case reports.

We report on three patients with postrenal acute kidney injury (AKI) showing a remarkably low level of cystatin C (CysC) compared with that of creatinine (Cr). The levels of Cr and CysC (Cr/CysC) were respectively as follows: 12.16 mg/dl/1.26 mg/l, 17.92 mg/dl/0.95 mg/l and 18.94 mg/dl/0.55 mg/l. The causes of urinary tract obstruction were benign prostatic hypertrophy, urinary bladder carcinoma and urethral stenosis due to radiation therapy for bladder carcinoma. Renal function was promptly recovered after relief of the obstruction. It is considered that the discrepancy strongly indicated AKI because of urinary tract obstruction and encouraged relief of the obstruction in order to recover renal function. Although the precise mechanism for the discrepancy was not determined, the maintenance of glomerular filtration and proximal tubular reabsorption of CysC long after the cessation of Cr excretion because of urinary tract obstruction seemed to be involved. This finding may be beneficial for the diagnosis and reversal of postrenal AKI and provides new insight into the process of postrenal AKI.