Direct radioimmunoassay for serum estriol during pregnancy

I describe an assay for serum estriol during pregnancy, which does not require prior extraction or chromatography of the serum. Fifty microliters of 20-fold diluted serum can be assayed to detect as little as 20 pg of estriol. For a 20 mug/liter serum estriol concentration the CV was 16%; for 100 mug/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 glucuronide at the 3' position, and a portion of the estriol 16 alpha (beta-D-glucuronide). Serum estriol concentrations increase during gestation from a low at 20 weeks of less than 10 mug/liter to a high of 100-150 mug/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.