Evaluation of cyclosporine kit utilizing an iodinated tracer and double antibody precipitation

Due to the increasing number of requests for Cyclosporine quantification, less time-consuming methods are being sought. A commercially available radioimmunoassay which utilizes an 125I-Cyclosporine tracer and double antibody precipitation to separate free and bound fraction is described. The assay was evaluated with standards prepared in-house as well as those supplied by the manufacturer. With both types of standards, the assay exhibited between-run coefficients of variation of less than 14.3%, recoveries near 100% and good parallelism. When the Cyclosporine values obtained with the 125I-RIA calibrated with standards provided in the kit were compared with those obtained with the RIA supplied by Sandoz, a relatively large proportional and constant error was noted. When the comparison was repeated, with the former assay being calibrated with standards prepared in-house, the magnitude of these errors was significantly reduced. With both types of standards the 125I-RIA exhibited good correlation with Cyclosporine values obtained by HPLC. This RIA is suitable for the relatively rapid quantification of Cyclosporine.