A direct radioimmunoassay was developed for assessing the levonorgestrel levels in human plasma without the need for heat, chromatographic or extraction pretreatment. D-norgestrel-3 (0-carboxymethyl)-oximino-BSA conjugate was used to raise antibodies having a low binding affinity towards other endogenous steroids. The rivanol-purified antiserum after dilution was used as the binding protein. The direct assay of levonorgestrel in human plasma was then compared to an extraction procedure. Lower intra-assay variability was shown by the direct assay when compared to the extraction method used at a sensitivity level of 0·15 nmol/L. The performance of the direct assay in quality control tests was more than favourable when compared with the extraction procedure. An examination of the effects of protein concentration on extraction efficiency was carried out together with an assessment of levonorgestrel levels in plasma in eight normal healthy women currently taking oral contraceptives and eight women who were not, at 0–24 h after the ingestion of 150 μg of D-NG and 30 μg of ethynyl oestradiol.
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