A Simplified Radioimmunoassay of Human Gravid Plasma Estriol

Abstract

A highly simplified and specific radioimmunoassay (RIA) method was developed to determine plasma unconjugated estriol (E₃) during pregnancy. The antigen used for immunization of rabbits was E₃-6-0-carboxymethyp-oxime-bovine serum albumin (E₃-6-oxime-BSA), a derivative of 6-oxo-E₃. At 24 weeks after the first immunization, the antiserum diluted 1: 50, 000 showed almost no cross reaction with neutral steroids and little cross reaction with phenolic steroids except for estrone-3-sulfate (32.6%).
Plasma samples were extracted with ethyl ether and evaporated to dryness. No chromatographic purification was necessary. The dried extracts were incubated with antiserum at 24°C for 30min, then antibody-bound and free E₃ were separated with dextran coated charcoal.
The sensitivity of this method was 10pg. The intra and inter coefficients of variation were 7.8% and 11.6%. The coefficient of variation of the present method was 12.8% compared to a method using a LH-20 column chromatography for purification.
Using our simplified method, the mean plasma E₃ during pregnancy was 2.6±1.4ng/ml at 16-20 weeks and 16.4±5.0ng/ml at term.