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

Abstract

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 EE₂ but 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 EE₂ were less than 25 pmol/l. Circulating concentrations of EE₂ could therefore be accurately determined in patients receiving low-dose combined preparations (EE₂ 35 μg; NE 500 μg).