A purification step prior to commercial sensitive immunoassay is necessary to achieve clinical usefulness when quantifying serum 17 beta-estradiol in prepubertal children

Objective: To test the clinical usefulness of sensitive commercial immunoassays for determination of low 17 beta-estradiol concentrations in children. Methods: The lower limit of detection and clinical usefulness (functional sensitivity) of three commercial estradiol immunoassays were validated by use of 500 sera from prepubertal and pubertal children and 55 pooled sera. The three immunoassays consisted of two modified direct immunoassays; one RIA (Spectria Estradiol RIA) and one time-resolved fluoroimmunoassay (AutoDELFIA Estradiol). both with increased serum volume in relation to antibody concentration and extended incubation time. In the third method, serum was purified and concentrated using diethyl ether extraction prior to measurement by the modified Spectria Estradiol RIA. Results: The lower limits of detection and clinical usefulness were 9 and 30 pmol/l for the direct RIA. 11 and 50 pmol/l for the AutoDELFIA, and 4 and 6 pmol/l for serum determined by extraction RIA. When measuring the serum pool originating from girls at breast stages 1-2, the direct RIA and AutoDELFIA resulted in significantly higher 17 beta-estradiol concentrations when compared with the extraction RIA (+58 and +267'%, P<0.001). We found a significant difference in 17 beta-estradiol concentrations between girls at breast stages 1 (median 6 pmol/l) and 2 (median 16 pmol/l), when quantified by the extraction RIA (P<0.0001) but no difference when quantified with the direct RIA (median values 12 and 14 pmol/l respectively). Conclusion: For determination of low serum 17 beta-estradiol concentrations in children, an extraction step prior to commercial immunoassay is needed to achieve clinically useful results.