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 cystati n C antibodies and horseradish peroxidase-conjugated antibodies) and s tudied the binding kinetics of antigen and antibodies. The within-assa y CV was <5%, the between-assay CV was 8.8%, the detection limit was 0 .9 mug/L, and the assay can be performed within 2 h. Cystatin C concen trations in sera from men were significantly higher than in women (mea n 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 dama ges to characterize the behavior of this low-M(r) protein as a possibl e indicator for estimating the glomerular filtration rate. The correla tion with the values obtained by a standard isotopic method involving Tc-99m-diethylenetriaminopentaacetic acid was r(s) = -0.89. The diagno stic sensitivity of cystatin C was 88.2% of that of the standard isoto pe clearance method and better than those of the conventional serum in dicators of reduced kidney function, beta2-microglobulin (64.7%) and c reatinine (52.9%).