Direct Radioimmunoassay for Serum Estriol during Pregnancy

I describe an assay for serum estriol during pregnancy, which does not require prior extraction or chromatogra-phy of the serum. Fifty microliters of 20-fold diluted serum can be assayed to detect as little as 20 pg of es-triol. For a 20.tg/liter serum estriol concentration the cv was 16 % ; for 1 00.tg/liter concentration it was 5 %. There was no significant interference by serum. ‘binding ” proteins. The assay measures unconjugated estriol, estriol conjugated with either sulfate or glucuro-nide at the 3 ’ position, and a portion of the estriol 16a-(f3-D-glucuronide). Serum estriol concentrations in-crease during gestation from a low at 20 weeks of <10 big/liter to a high of 100-150 j.tg/liter at term. Serum estriol values correlate well with those for urinary total estrogens and with values for human placental lactogen in serum at various gestational ages