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Aromatase inhibitors (AIs) reduce the production of estrogen and are used in postmenopausal women with hormone-receptor–positive breast cancer. AI therapy can reduce already low E2 concentrations in these patients to <1 pg/mL.^1,2 AI therapy failure, on the other hand, is associated with E2 concentrations in the 5–20 pg/mL range.³ Therefore, E2 is used as a potential biomarker to guide treatment decisions in these patients.

The most common methods for measuring E2 are immunoassays and liquid chromatography-mass spectrometry (LC-MS) methods. The lower limit of quantitation (LLOQ) of immunoassays is approximately 5–30 pg/mL compared to <1–5 pg/mL for LC-MS.^4,5 For a laboratory to be certified by the CDC Hormone Standardization Program, the total allowable error of its estradiol assay must be ± 2.5 pg/mL for samples ≤ 20 pg/mL.⁶ This is problematic for immunoassays, which generally have relatively high LLOQs. In addition, immunoassays demonstrate positive bias, according to results reported in the CAP Accuracy-Based Programs Survey. Finally, immunoassays are subject to interference by drugs such as fulvestrant and the aromatase inhibitor exemestane.^7,8 In summary, LC-MS assays are preferred for populations with low E2 concentrations.

1. Dixon JM, Renshaw L, Young O, et al. Letrozole suppresses plasma estradiol and estrone sulphate more completely than anastrozole in postmenopausal women with breast cancer. J Clin Oncol. 2008;26(10):1671–1676.
2. Handelsman DJ, Gibson E, Davis S, Golebiowski B, Walters KA, Desai R. Ultrasensitive serum estradiol measurement by liquid chromatography-mass spectrometry in postmenopausal women and mice. J Endocr Soc. 2020;4(9):bvaa086.
3. Faltinová M, Vehmanen L, Lyytinen H, et al. Monitoring serum estradiol levels in breast cancer patients during extended adjuvant letrozole treatment after five years of tamoxifen: a prospective trial. Breast Cancer Res Treat. 2021;187(3):769–775.
4. Bertelsen BE, Kellmann R, Viste K, et al. An ultrasensitive routine LC-MS/MS method for estradiol and estrone in the clinically relevant sub-picomolar range. J Endocr Soc. 2020;4(6):bvaa047.
5. Nagao T, Kira M, Takahashi M, et al. Serum estradiol should be monitored not only during the peri-menopausal period but also the post-menopausal period at the time of aromatase inhibitor administration. World J Surg Oncol. 2009;7:88.
6. HoSt/VDSCP: Certified Participants. Centers for Disease Control and Prevention. https://www.cdc.gov/labstandards/hs_certified_participants.html
7. Owen LJ, Monaghan PJ, Armstrong A, et al. Oestradiol measurement during fulvestrant treatment for breast cancer. Br J Cancer. 2019;120(4):404–406.
8. Mandic S, Kratzsch J, Mandic D, et al. Falsely elevated serum oestradiol due to exemestane therapy. Ann Clin Biochem. 2017;54(3):402–405.

Brian Harry, MD, PhD
Assistant Professor of Pathology
University of Colorado Anschutz Medical Campus
Aurora, Colo.
Member, CAP Accuracy-Based Programs Committee

Joely Straseski, PhD, DABCC
Professor of Pathology
University of Utah School of Medicine
Section Chief, Clinical Chemistry
Medical Director, Endocrinology
ARUP Laboratories
Salt Lake City, Utah
Member, CAP Accuracy-Based Programs Committee