A radioimmunoassay for total captopril in human serum or plasma samples

A radioimmunoassay for the measurement of total captopril in serum and heparinized plasma has been developed. Prior to the assay, serum or heparinized plasma samples are subjected to tri-n-butyl-phosphine reduction followed by N-ethylmaleimide (NEM) derivatization. The assay utilizes in-house NEM-captopril antibody, [125I]NEM-captopril radiolabel and human serum standards. Satisfactory zero binding and sensitivity are obtained after 3 h of incubation at room temperature. Separation of the antibody-bound and free radiolabeled antigen is achieved by employing a polyethylene glycol solution. The assay was shown to have excellent parallelism, recovery, and precision. Cross-reactivities with potentially interfering substances were low.