Sandwich enzyme immunoassay of cystatin C in serum with commercially available antibodies

We developed a sandwich enzyme immunoassay for determining cystatin C in serum by using commercially available antibodies. We optimized each assay step (e.g., concentrations of coating rabbit anti-human cystatin C antibodies and horseradish peroxidase-conjugated antibodies) and studied the binding kinetics of antigen and antibodies. The within-assay CV was < 5%, the between-assay CV was 8.8%, the detection limit was 0.9 microgram/L, and the assay can be performed within 2 h. Cystatin C concentrations in sera from men were significantly higher than in women (mean and SD: 2.14 +/- 0.31 vs 1.78 +/- 0.26 mg/L). We studied the cystatin C concentrations in sera of 31 outpatients with suspected kidney damages to characterize the behavior of this low-M(r) protein as a possible indicator for estimating the glomerular filtration rate. The correlation with the values obtained by a standard isotopic method involving 99mTc-diethylenetriaminopentaacetic acid was rs = -0.89. The diagnostic sensitivity of cystatin C was 88.2% of that of the standard isotope clearance method and better than those of the conventional serum indicators of reduced kidney function, beta 2-microglobulin (64.7%) and creatinine (52.9%).