Cyclosporine blood concentrations determined by specific versus nonspecific assay methods

Cyclosporine blood concentrations were simultaneously determined by radioimmunoassay (RIA) and high-performance liquid chromatography (HPLC) at multiple points in time in two patients receiving cyclosporine for immunosuppression following liver transplantation. Radioimmunoassay levels always exceed those determined by HPLC; however, the divergence between the two methods increased as serum bilirubin concentration increased, with HPLC:RIA ratios generally less than 0.3 when serum bilirubin concentrations exceeded 10.0 mg/dL. These preliminary results suggest that retention of immunoactive cyclosporine metabolites due to imparied liver function may account for RIA-determined cyclosporine concentrations that greatly exceed those measured by HPLC.