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Comparison of Methods to Measure Low Serum Estradiol Levels in Postmenopausal Women

San Francisco Coordinating Center (J.S.L., S.R.C.), California Pacific Medical Center Research Institute, San Francisco, California 94107; Division of Endocrinology and Metabolism (J.S.L.), San Francisco General Hospital, Department of Epidemiology and Biostatistics and Women’s Health Clinical Research Center (E.V., D.G.), and Division of General Internal Medicine, Department of Medicine (M.S.B.), University of California, San Francisco, California 94143; Division of Research (B.E.), Kaiser Permanente Medical Care Program, Northern California, Oakland, California 94612; Departments of Obstetrics and Gynecology, and Preventive Medicine (F.Z.S.), University of Southern California Keck School of Medicine, Los Angeles, California 90033; Berlex Laboratories, Inc. (V.H.), Montville, New Jersey 07045; Department of Epidemiology (J.A.C.), University of Pittsburgh, Pittsburgh, Pennsylvania 15261; Esoterix Endocrinology Inc. (W.C.), Calabasas, California 91301; SFBC Taylor Technology (J.S.), Inc., Princeton, New Jersey 08540; and Department of Biochemistry (E.F., M.D.), Royal Marsden Hospital, London SW3 6JJ, United Kingdom

Address all correspondence and requests for reprints to: Jennifer S. Lee, M.D., Division of Endocrinology, Clinical Nutrition, and Vascular Medicine, University of California Davis Medical Center, 4150 V Street, Suite G400, Sacramento, California 95817. E-mail: catechols{at}gmail.com.

Context: Accurate measurement of low serum estradiol (E₂ < 30 pg/ml or < 110 pmol/liter) is needed to study relationships between endogenous E₂ and risks of diseases in older women.

Objective: 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).

Design/Setting: In a pilot study of 40 postmenopausal women, endogenous E₂ 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.

Main Outcomes: Pearson correlation coefficients among E₂ measurements by assay methods and BMI, and their confidence intervals, by bias-corrected bootstrap method, were used.

Results: In the pilot study, E₂ 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 E₂ by the indirect and direct assays were higher (by 14 and 68%, respectively) and less reproducible than by GC-MS/MS.

Conclusion: Until mass spectrometry is practical for wide use, extraction-based indirect assays may be preferable for measuring low postmenopausal serum E₂.

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