A simple radioimmunoassay for unconjugated estriol in pregnancy plasma

A simple and specific radioimmunoassay (RIA) for the measurement of plasma or serum unconjugated estriol (E3) in pregnancy is described for general laboratory use. Two different antibodies utilized had low cross-reaction to estrone (E1) and estradiol-17 beta (E2). With either of these antibody reagents, a method was designed that requires only an extraction step using a specific solvent mixture of ethyl acetate and n-hexane (3:2, v/v), one-hour incubation with working antibody, and separation of bound from free E3 by ammonium sulfate. Standard curves useful in the range of 0 to 2,000 pg. were obtained. This assay, requiring only 0.05 ml. of plasma, can be completed within 4 hours. The sensitivity of this RIA was 10 pg. The intra- and interassay coefficients of variation (CV) were 5.0 and 9.4 per cent, respectively. When E3 determinations by this method are compared with RIA values by a more complicated technique using Celite column chromatography, the results are not significantly different. There was a good correlation between our RIA and a standard urine E3 determination (n = 137, r = 0.82, p less than 0.001). Using our simplified method, the mean plasma E3 during pregnancy were 1.0 +/- 0.6 ng. per milliliter at 10 weeks, 2.3 +/- 0.6 ng. per milliliter at 20 weeks, 6.5 +/- 1.4 ng. per milliliter at 30 weeks, and 16.5 +/- 5.1 ng. per milliliter at term. Since the serum RIA reflects changes in free E3 which has a shorter half-life than conjugated E3, this rapid and simple method is attractive as an approach to monitor fetal-placental function.