On the validity of a levo-norgestrel radioimmunoassay

The validity of a levo-norgestrel (L-NOG) radioimmunoassay in human plasma was assessed using a test of radiochemical purity. Extraction with diethyl ether followed by chromatography on celite gave an L-NOG peak of constant specific activity, indicating the radiochemical purity of this compound. This was demonstrated with three different plasma pools. Hence, the chromatographic method was considered to yield valid results. When diethyl ether or petroleum ether extracts were assayed without chromatography, overestimates of 37% and 18%, respectively, were encountered.