Radioimmunoassay for etiocholanolone

A sensitive and reliable radioimmunoassay for serum unconjugated etiocholanolone (3alpha-hydroxy-5beta-androstan-17-one) is reported. The antiserum was obtained from rabbits by immunization of etiocholanolone-17-(O-carboxymethyl) oxime [CMO]-bovine serum albumin [BSA]. Two ml of serum with 3H-etiocholanolone added for recovery was extracted with ether, and etiocholanolone was separated from cross-reacting steroids by Sephadex LH-20 column chromatography. The mean recovery after extraction and chromatography was 80.7 +/- 6.8 (S.D.)%. The sensitivity of the assay was less than 40 pg. The intraassay and interassay coefficients of variation were 9.2% and 10.9%, respectively. The mean of serum unconjugated etiocholanolone concentration determined by the present method was 0.39 +/- 0.10 (S.D.) ng/ml (n = 50) in normal men and 0.36 +/- 0.08 (S.D.) ng/ml (n = 20) in women in the follicular phase of the menstrual cycle.