A Radioimmunoassay for Ethinyl Oestradiol in Plasma Incorporating an Immunosorbent, Pre-assay Purification Procedure

A radioimmunoassay for plasma ethinyl oestradiol, featuring an immunosorbent extraction procedure, is described. Ethinyl oestradiol (EE2) was extracted using a non-specific, anti-oestrogen serum, raised to an oestradiol-17-hemisuccinate conjugate. The antiserum, coupled to microcrystalline cellulose, selectively extracted EE2but not norethisterone (NE), thus conferring specificity on a radioimmunoassay which has previously exhibited unacceptably high cross-reactivity with the synthetic progestagen, norethisterone, often used concomitantly with ethinyl oestradiol. This radioimmunoassay was shown to fulfil accepted assay validation criteria. Levels in subjects not receiving EE2were less than 25 pmol/l. Circulating concentrations of EE2could therefore be accurately determined in patients receiving low-dose combined preparations (EE235 μg; NE 500 μg).