Is the monoclonal fluorescence polarization immunoassay for cyclosporine specific? Comparison with specific radioimmunoassay.

Trough concentrations of cyclosporine (CsA) in whole blood were measured by specific monoclonal 3H-radioimmunoassay (S-RIA) and compared with those obtained by monoclonal fluorescence polarization immunoassay (S-FPIA) in 89 transplant recipients. Differences in the S-RIA:S-FPIA relationship between kidney transplant (KT, n = 59) and heart transplant (HT, n = 24) recipients were investigated. Mean concentrations of CsA were significantly higher by S-FPIA than by S-RIA [191 +/- 127 vs. 166 +/- 124 ng/ml (mean +/- SD), p less than 0.001], with a S-FPIA:S-RIA ratio of 1.25 +/- 0.33. Higher ratios were observed either in patients with low CsA concentrations or in those with a high proportion of metabolites. HT recipients had higher S-FPIA:S-RIA ratios than did KT recipients (1.37 vs. 1.22, t = 1.87, p = 0.065). Within-assay coefficients of variation were lower for S-FPIA than for S-RIA (2.4% vs. 12.3%, p less than 0.001). Monoclonal FPIA is a precise and fast method, more suitable than RIA for therapeutic CsA monitoring in clinical practice. However, our results indicate a 25% higher mean CsA concentration by S-FPIA than by S-RIA (37% in HT recipients), which should be borne in mind until therapeutic and toxic ranges are established or a more specific FPIA method is developed.