Accurate measurement of low serum estradiol (E(2) < 30 pg/ml or < 110 pmol/liter) is needed to study relationships between endogenous E(2) and risks of diseases in older women. The objective of this study was to determine whether an extraction-based (indirect) assay or a non-extraction-based (direct) assay correlates better with mass spectrometry and body mass index (BMI). In a pilot study of 40 postmenopausal women, endogenous E(2) measurements from three indirect and four direct assay methods and gas chromatography-tandem mass spectrometry (GC-MS/MS) were compared. A confirmatory study compared an indirect and a direct assay, selected among those in the pilot study, to GC-MS/MS; this study was conducted in 374 postmenopausal women not taking hormone therapy from the Ultra Low-dose TRansdermal estrogen Assessment (ULTRA) trial. Pearson correlation coefficients among E(2) measurements by assay methods and BMI, and their confidence intervals, by bias-corrected bootstrap method, were used. In the pilot study, E(2) by three indirect assays correlated better (P < 0.03) with GC-MS/MS and with BMI than measurements by four direct assays. In the confirmatory study, the indirect assay correlated better (P < 0.01) with GC-MS/MS and BMI than the direct assay. Measurements by the indirect and direct assays were overestimated, but deviations in direct assay measurements were less precise. Mean E(2) by the indirect and direct assays were higher (by 14 and 68%, respectively) and less reproducible than by GC-MS/MS. Until mass spectrometry is practical for wide use, extraction-based indirect assays may be preferable for measuring low postmenopausal serum E(2).

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