A direct radioimmunoassay (RIA) for the quantitation of Retrovir® (zidovudine, azidothymidine, AZT) in biological fluids has been developed. The assay is sensitive with an I50value of about 30 nM and with a lower limit of detection of about 3 nM. Intra-assay precision gave sample coefficients of variation that ranged from 1.77 to 8.65% for the standard curve with human plasma. Inter-assay precision and accuracy were within acceptable limits. The RIA was validated by comparing results obtained from the analysis of rat plasma samples by both this RIA and a high-performance liquid chromatography method. None of the crossreactivities recorded should interfere with the assay system. The affinity constant of the antibody chosen for use was 1.4 × 109L/mol.
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