title: Comparison of methods to measure low serum estradiol levels in postmenopausal women.
abstract: BACKGROUND
  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.
  
  
  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).
  
  
  METHODS
  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.
  
  
  RESULTS
  Pearson correlation coefficients among E(2) measurements by assay methods and BMI, and their confidence intervals, by bias-corrected bootstrap method, were used.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  Until mass spectrometry is practical for wide use, extraction-based indirect assays may be preferable for measuring low postmenopausal serum E(2).
authors.affiliation: San Francisco Coordinating Center, California Pacific Medical Center Research Institute, San Francisco, CA 94107, USA. catechols@gmail.com
authors.forename: Jennifer S
authors.initials: JS
authors.lastname: Lee
authors.forename: Bruce
authors.initials: B
authors.lastname: Ettinger
authors.forename: Frank Z
authors.initials: FZ
authors.lastname: Stanczyk
authors.forename: Eric
authors.initials: E
authors.lastname: Vittinghoff
authors.forename: Vladimir
authors.initials: V
authors.lastname: Hanes
authors.forename: Jane A
authors.initials: JA
authors.lastname: Cauley
authors.forename: Walt
authors.initials: W
authors.lastname: Chandler
authors.forename: Jim
authors.initials: J
authors.lastname: Settlage
authors.forename: Mary S
authors.initials: MS
authors.lastname: Beattie
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Folkerd
authors.forename: Mitch
authors.initials: M
authors.lastname: Dowsett
authors.forename: Deborah
authors.initials: D
authors.lastname: Grady
authors.forename: Steven R
authors.initials: SR
authors.lastname: Cummings
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D015992:Body Mass Index; D004958:Estradiol / Q000097:blood*; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D008875:Middle Aged; D010865:Pilot Projects; D017698:Postmenopause / Q000097:blood*
chemical_list: D004958:Estradiol