Radioimmunoassay of 15alpha-hydroxyestriol (estetrol) in pregnancy serum

A more specific antisera against 15alpha-hydroxyestriol (estetrol) was produced in rabbits by immunization with 6-oxoestetrol-6-carboxy-methyloximino-bovine serum albumin. The synthesis of estetrol tetraacetate was greatly improved. Characterization of estetrol antisera revealed that it is more specific than those formerly obtained. A simple method of extraction of estetrol with alcohol from serum with high recovery is described. A radioimmunoassay method for pregnancy serum is characterized in regard to its specificity, accuracy, sensitivity and precision. The method is able to measure as little as 50 pg/tube or 167 pg/ml of serum. The intraassay coefficient of variation is 7.8% and that for interassay including personnel variation is 14.2%. Accuracy of the method, as determined by the recovery of non-radioactive estetrol (1 ng/ml) added to non-pregnancy serum, is found to be 89 +/- 8 (SD)%, CV = 8.7%, without correction for experimental losses. One hundred twelve pregnancy sera were analysed; the values obtained from normal subjects in the third trimester ranged from 240-950 pg/ml. The values obtained by this method are somewhat lower than those reported previously, presumably due to the higher specificity of the antibody.