Three commercial polyclonal immunoassays for cyclosporine in whole blood compared: 2. Cross-reactivity of the antisera with cyclosporine metabolites.

We demonstrate the diverse selectivity of three commercial polyclonal "cyclosporine" immunoassays for cyclosporin (CsA) metabolites by comparing analytical responses of nine metabolites added to drug-free whole-blood specimens (range 0 to 2000 micrograms/L) and assayed by the Abbott TDx fluorescence polarization immunoassay (FPIA), the Incstar Cyclo-Trac radioimmunoassay (RIA), and the Sandoz RIA. Cross-reactivity--defined as the relative response (slope of regression line) of metabolite/parent CsA over the assay's linear range of concentrations--differed for each metabolite among the three assays. Overall, Abbott's antiserum exhibited the greatest affinity for the metabolites, the Sandoz antiserum the least. Ranges of cross-reactivity for the metabolites over all three assays were M1 (14-44%), M8 (9-20%), M13 (13-26%), M17 (50-116%), M18 (17-79%), M21 (4-54%), M25 (less than 1-52%), M26 (less than 1-29%), and M203-218 (7-51%). The specificities of the Abbott, Incstar, and Sandoz polyclonal assays thus differ significantly, and this brings into question the practical utility of comparing data generated for patients' specimens by different procedures.