Fulvestrant Interference with Estradiol Immunoassays: A Case Report and Review of Laboratory Implications
https://doi.org/10.15605/jafes.041.S1
- Author:
Dywei Lee
1
;
Waye Hann Kang
1
Author Information
1. Department of Medicine, M. Kandiah Faculty of Medicine and Health Sciences, Universiti Tunku Abdul Rahman
- Publication Type:Journal Article
- MeSH:
Fulvestrant;
Immunoassay
- From:
Journal of the ASEAN Federation of Endocrine Societies
2026;41(S1):101-
- CountryPhilippines
- Language:English
-
Abstract:
Introduction:Fulvestrant, a selective estrogen receptor degrader
widely used in hormone receptor-positive breast
cancer, can cause significant analytical interference with
estradiol immunoassays due to structural similarity with
17β-estradiol. This interference may result in falsely
elevated estradiol measurements, potentially leading
to diagnostic confusion and inappropriate clinical
interventions. We report a case demonstrating this
clinically significant analytical interference and emphasize
the importance of liquid chromatography-tandem mass
spectrometry (LC-MS/MS) in hormone monitoring for
patients receiving fulvestrant therapy.
Case:A 31-year-old female with metastatic hormone receptorpositive breast cancer receiving combination therapy
(fulvestrant, letrozole, ribociclib, and goserelin) presented
with inappropriately elevated serum estradiol levels of 566
pmol/L (early follicular phase 200–500 pmol/L), measured
by chemiluminescence immunoassay. Despite effective
gonadotrophin suppression evidenced by low luteinizing
hormone (0.2 IU /L) and follicle-stimulating hormone
(4.1 IU/L) levels and clinical amenorrhea, the discordant
estradiol elevation raised suspicion of assay interference.
Confirmatory testing with LC-MS/MS revealed an estradiol
concentration of <36 pmol/L, consistent with expected
hormonal suppression and confirming immunoassay
cross-reactivity with fulvestrant.
The case demonstrates significant fulvestrant interference
with estradiol immunoassays, resulting in a greater than 15-
fold overestimation compared to LC-MS/MS measurement.
The clinical presentation was consistent with effective
endocrine therapy, supported by suppressed gonadotropin levels, clinical amenorrhea, and stable disease on imaging.
LC-MS/MS provided accurate estradiol measurement,
avoiding potential diagnostic confusion and unnecessary
clinical interventions.
Conclusion:Clinicians should be aware of potential fulvestrant interference with estradiol immunoassays when interpreting
hormone levels in breast cancer patients. Discordant
laboratory findings, particularly elevated estradiol despite
clinical evidence of effective hormonal suppression, should
prompt consideration of LC-MS/MS confirmation. This
case underscores the clinical importance of analytical
method selection in hormone monitoring and highlights
the need for improved laboratory-clinical communication
to optimize patient care in oncology practice.
- Full text:2026080416421399995EP_A146.pdf
