Cyclosporine A monitoring in patients with renal, cardiac, and liver transplants: a comparison between fluorescence polarization immunoassay and two different RIA methods.

In the present study a new method for selectively determining parent cyclosporine (CsA) in whole blood, a fluorescence polarization immunoassay (FPIA; TDx Abbott), was compared with a RIA method (Sandimmun, Sandoz Ltd, Basle, Switzerland). A total of 974 samples were collected during the first 3 post-operative months from 63 renal, cardiac, and liver transplant recipients. The CsA concentrations measured with FPIA ranged from 14% to 19% above RIA (specific) in the middle ranges. Regression equations in renal transplants: FPIA = 1.001 x RIA + 28; in heart transplants: FPIA = 1.08 x RIA + 27 and in liver transplants: FPIA = 1.13 x RIA + 13. Considering the improved precision of the new method (inter-assay CV with FPIA: 3.8-9.5%; with RIA: 18.6%), the slightly lower specificity will usually be of minor importance in the therapeutic range for whole blood CsA concentrations following organ transplantations. The FPIA measurements which deviated most from the regression line compared with RIA-specific CsA values, tended to coincide with high CsA concentrations or rather extreme RIA specific to RIA non-specific ratios. In addition to analytical imprecision with the RIA-specific method, lower specificity of the FPIA vs. some of the metabolites may explain these deviations. The majority of these observations occurred as isolated episodes with normal relationship between RIA specific and FPIA on preceding and following days. Accordingly large dosage adjustments should await verification in repeated samples. Following these precautions the FPIA method may prove useful and safe in the monitoring of cyclosporine treatment.