Evaluation of a radioimmunoassay for estradiol in unextracted serum

We describe the performance of a commercial (Steranti/EIR) RIA reagent kit for measuring 17 beta-estradiol directly in serum. Day-to-day precision data for control sera were as follows: mean = 102.8 ng/L, CV = 6.8%, n = 20; mean = 231.1 ng/L, CV = 5.3%, n = 21; mean = 747.7 ng/L, CV = 9.4%, n = 21. Analytical recovery of added estradiol from seven different serum pools from men, to which three different concentrations of estradiol had been added, was (mean +/- SD): 98.6 +/- 7.0% at 107.5 ng/L added; 98.8 +/- 4.7% at 322.5 ng/L added; 108.2 +/- 4.8% at 645 ng/L added. Overall recovery of estradiol in these experiments (mean +/- SD for 21 determinations) averaged 101.9 +/- 7.0%. Assay of 32 serum specimens from women by both the direct (y) and an extraction method (x) gave the following linear regression statistics: y = 1.12x - 1.3, r = 0.998, Sy/x = 30.2 ng/L, mean y = 438.2 ng/L, mean x = 391.4 ng/L. Hemoglobin, bilirubin, and moderate lipemia do not interfere. Sensitivity of the direct assay was 2.6 ng/L. Compared with the extraction assay, the direct estradiol assay has advantages of speed and simplicity.