Evaluation of a radioimmunoassay for serum unconjugated estriol using commercial reagents

A radioimmunoassay using a commercially available antiserum was evaluated for measurement of serum unconjugated estriol in pregnancy. The evaluation showed an inter-assay variance of 12.1%, intra-assay variance of 6.8%, sensitivity of 0.2-0.6 ng/ml (0.7-2.1 nmol/l), and average recovery of 85.3%. The assay is specific for unconjugated estriol, showing less than 1% cross-reactivity with estriol-3-sulfate and estriol-16-glucuronide. Normal limits were established from 7 to 40 weeks' gestation using 175 serum samples. No diurnal variation could be demonstrated at 8 a.m. and 3 p.m. Eighty-nine serum specimens and 82 urine specimens obtained from 18 high-risk pregnancies were within normal limits except in cases of intruterine fetal death, pre-eclampsia, and suspected placental sulfatase deficiency. Serial urinary estriol levels fluctuated as much as 75%, while serial serum samples varied by only 30%.