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Submitted on October 31, 2005
Accepted on July 25, 2006
San Francisco Coordinating Center, California Pacific Medical Center Research Institute, San Francisco, California; Division of Endocrinology and Metabolism, San Francisco General Hospital, University of California, San Francisco, California; Division of Research, Kaiser Permanente Medical Care Program, Northern California; Departments of Obstetrics and Gynecology, and Preventive Medicine, University of Southern California Keck School of Medicine, Los Angeles, California; Department of Epidemiology and Biostatistics and Women's Health Clinical Research Center, University of California, San Francisco, California; Berlex Laboratories, Inc., Montville, New Jersey; Department of Epidemiology, University of Pittsburgh, Pittsburgh, Pennsylvania; Esoterix Endocrinology Inc., Calabassas, California; SFBC Taylor Technology, Inc., Princeton, New Jersey; Division of General Internal Medicine, Department of Medicine, UC San Francisco, California; Department of Biochemistry, Royal Marsden Hospital, London, United Kingdom
* To whom correspondence should be addressed. E-mail: catechols{at}gmail.com.
Context: Accurate measurement of low serum estradiol (E2 < 30 pg/ml or < 110 pmol/L) is needed to study relationships between endogenous E2 and risks of diseases in older women.
Objective: 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 3 indirect and 4 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 3 indirect assays correlated better (P < 0.03) with GC-MS/MS and with BMI than measurements by 4 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 over-estimated, 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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