Direct radioimmunoassay of progesterone in bovine plasma using danazol (17-alpha-2,4-pregnadien-20-yno(2,3-d)isoxazol-17-ol) as a displacing agent.

The majority of progesterone in plasma is bound to cortisol-binding globulin and albumin carrier proteins. In the determination of plasma progesterone concentration by radioimmunoassay (RIA), it is necessary to remove these carrier proteins or displace the hormone from them. In the present study, we have examined the suitability of danazol (17-alpha-2,4-pregnadien-20-yno(2,3-d)isoxazol-17-ol), a synthetic steroid, to displace progesterone from plasma proteins in a direct RIA of bovine plasma. Accordingly, the progesterone content of bovine plasma samples was measured with a RIA using danazol as a displacing agent (direct RIA) and compared with results obtained with a RIA incorporating a preliminary solvent extraction step (extraction RIA). Danazol did not alter the standard curve for progesterone. Sensitivity (ED80) of the direct RIA (9.8 pg/tube) was comparable to that of the extraction RIA (10.3 pg/tube). Results for progesterone assayed in the direct RIA correlated well (r = 0.99) with the results obtained with the extraction RIA. The direct RIA was shown to be accurate; the mean recovery of known amounts of progesterone added to a sample of pooled bovine plasma was 98.5% +/- 3.29 (SEM). The direct RIA intra-assay coefficient of variation (CV) RIA for samples within the low concentration range (0.1-1.0 ng/mL), the medium concentration range (1.0-3.0 ng/mL) and the high concentration range (3.0-6.0 ng/mL) of progesterone were 8.1%, 8.3% and 7.73%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)