[Evaluation of urinary cystatin C as a marker of renal dysfunction]

Urinary excretion of some low molecular weight proteins (LMWPs) is used as an indicator of tubular dysfunction, since they are increased by the damage of tubular reabsorption. Although serum cystatin C is known to be a sensitive marker for GFR, the property of urinary cystatin C as a LMWP has not been fully observed. We evaluated the clinical utility of urinary cystatin C.

Urine samples were collected from 130 patients with various degree of renal dysfunction, 62 healthy subjects, and 2 patients with acute renal failure, one with renal acute renal failure, the other with prerenal acute renal failure. Urine levels of cystatin C, beta2-microglobulin (beta2mG), and alpha1-microglobulin (alpha1mG) were measured by immunonephelometry. Creatinine clearance(Ccr) tests were conducted on 130 patients with renal dysfunction. Creatinine(CRE) was measured by enzyme assay.

The daily urinary excretions of cystatin C and alpha1mG were increased significantly in patients with Ccr<30 ml/min(group I), compared to those in patients with 30 < or = Ccr<70 ml/min(II), and Ccr > or = 70ml/min(III). Although the mean daily excretion of beta2mG increased as Ccr decreased, the significant difference was not observed. The rate of increase in the mean value between III and I was extremely high in cystatin C. Fractional excretions of cystatin C and beta2mG calculated in the same groups increased significantly in I compared to II and III. The rate of increase in the mean value was higher in cystatin C. Regression analyses between urine CRE and each three LMWP gave the best correlation coefficient for cystatin C in healthy subjects. While in one patient with renal acute renal failure, the rate of increase in urine cystatin C was higher than that of other LMWPs, in another patient with prerenal acute renal failure, the rate of increase in urine cystatin C was low.

Although details of urinary movement of LMWPs in nephrons have not been clearly elucidated, the urinary cystatin C seems to have distinctive properties, and to be useful for the evaluation of renal injury.