This assay was originally developed for the World Health Organization. It has been widely used in clinical trials and pharmacokinetic studies of contraceptive preparations and hormone replacement therapy (HRT) delivery systems both by the World Health Organization and by major pharmaceutical companies. The method is an extraction RIA and not subject to the well- documented interference problems associated with direct serum/plasma immunoassays.

Norethisterone in the sample (100µL of serum or plasma) is extracted with diethyl ether. The extract is dried and redissolved in assay buffer. The redissolved extract is assayed for Norethisterone by 3H-RIA RIA (tritium labelled radioimmunoassay with charcoal separation). All samples, standards and quality control specimens are assayed in duplicate.

The assay is controlled through the use of at least 3 Internal Quality Control samples in every run. In addition the performance of every run is closely monitored for various parameters including imprecision, antibody binding of tracer, non-specific binding and reproducibility of the standard curve.

• Standard Curve: The range of the standard curve is 0 to 6000 pmol/L (1800 pg/mL).
• Precision: The between-run precision of the assay is generally less than 10% CV.
• Sensitivity: The assay reliably measures concentrations as low as 190 pmol/L (57 pg/mL).

The cross-reactions of the assay (estimated as the relative potency causing a 50% displacement of zero binding) for each substance tested are shown below:

Substance % Cross Reaction Norethisterone 100 5a-dihydronorethisterone 31.2 5b-dihydronorethisterone 23.2 3b,5a-tetrahydronorethisterone 19.3 3b,5b-tetrahydronorethisterone 12.9 3a,5a-tetrahydronorethisterone 5.0 3a,5b-tetrahydronorethisterone 1.6 Ethinyl Estradiol 2.7 Testosterone 0.2 Dihydrotestosterone 0.1 Norethisterone enanthate <0.1

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