Cyclosporine Blood Concentrations Determined by Specific Versus Nonspecific Assay Methods

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Abstract
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 impaired liver function may account for RIA‐determined cyclosporine concentrations that greatly exceed those measured by HPLC.