How Fulvestrant Treatment Can Skew Estrogen Blood Test Results: What Breast Cancer Patients Need to Know

Table of Contents

• Key Points
• Background: Why Accurate Estrogen Testing Matters
• Study Methods: How the Research Was Conducted
• Key Findings: What the Tests Revealed
• Discussion: Understanding the Interference Problem
• Clinical Implications: What This Means for Patients
• Limitations: What This Study Could Not Prove
• Recommendations: What Patients and Doctors Should Do
• Frequently Asked Questions
• Source Information

Key Points

• In a study of 10 women taking fulvestrant, two common immunoassays falsely showed pre-menopausal estrogen levels in 100% and 80% of patients.
• Mass spectrometry (LC-MS/MS) correctly showed all 10 women on fulvestrant had post-menopausal estrogen levels, with a detection limit far lower than immunoassays.
• Fulvestrant interference can persist for months after the last dose; two patients had falsely elevated readings 2 and 4 months later.
• Standard immunoassays are not sensitive enough for post-menopausal estrogen levels, overestimating low concentrations even in women without cancer.
• Researchers recommend using LC-MS/MS instead of immunoassays to assess menopausal status in women currently or recently treated with fulvestrant.

Background: Why Accurate Estrogen Testing Matters

Determining whether a woman is pre- or post-menopausal is a critical step in planning treatment for oestrogen receptor (ER)-positive breast cancer — the most common type of breast cancer, in which tumour cells grow in response to oestrogen. Many clinical trials of endocrine (hormone-blocking) therapy require strict confirmation of menopausal status before a woman can enrol. This confirmation typically includes measuring the level of oestradiol (the main form of oestrogen) in the blood.

Most hospital laboratories use immunoassays (IAs) — antibody-based tests — for this purpose. However, there is a well-known problem: oestradiol concentrations in most post-menopausal women are actually below the detection limit of these commercial immunoassays. The tests simply are not sensitive enough to reliably measure such tiny amounts.

An additional and more alarming issue emerged in 2016. Manufacturers of commercial immunoassays released Field Safety Notices describing a significant cross-reactivity problem with fulvestrant (brand name Faslodex), a drug used to treat advanced ER-positive breast cancer. Fulvestrant is a selective oestrogen receptor degrader — it works by binding to oestrogen receptors on cancer cells and causing the receptors to be destroyed.

Because fulvestrant and oestradiol have very similar chemical structures, antibody-based oestradiol tests can mistakenly identify fulvestrant as oestradiol. This produces falsely elevated oestradiol readings. Regulatory bodies — including the US Food and Drug Administration (FDA) and the UK's Medicines and Healthcare products Regulatory Agency (MHRA) — issued medical device alerts urging caution. The Summary of Product Characteristics for fulvestrant was updated to state: "Due to the structural similarity of fulvestrant and oestradiol, fulvestrant may interfere with antibody based-oestradiol assays and may result in falsely increased levels of oestradiol."

Despite these warnings, the researchers behind this study were concerned that clinical awareness of the issue remained limited. As a result, women could still be getting inappropriate suppression of ovarian function — or being excluded from clinical trials — based on inaccurate test results.

Study Methods: How the Research Was Conducted

To investigate the extent of the problem, the team conducted a service evaluation — an assessment of existing laboratory practice used to improve services rather than a controlled experiment. They compared three different laboratory methods for measuring oestradiol, including two immunoassays currently used in their hospitals in Manchester, UK, and one high-precision mass spectrometry method as the "gold standard."

The Three Tests Compared

• Siemens Centaur XP (SC): A commonly used immunoassay with a lower limit of quantitation (LLOQ — the lowest concentration it can accurately measure) of 70 pmol/L.
• Abbott Architect (AA): Another widely used immunoassay with an LLOQ of 92 pmol/L.
• LC-MS/MS (liquid chromatography-tandem mass spectrometry): A highly sensitive and specific analytical method with an LLOQ of just 10 pmol/L, meaning it can detect far lower levels of oestradiol than the immunoassays.

The LC-MS/MS assay was calibrated using certified reference materials, and matrix-matched reference materials were used to validate the calibrators — ensuring the accuracy of the gold-standard method. The study also verified test precision, reporting coefficients of variation (CVs) of less than 8% for Siemens, less than 9.2% for Abbott, and less than 5.1% for LC-MS/MS. Lower CVs indicate more consistent, reproducible results.

The Patient Groups Studied

Three groups of women were included in the evaluation:

1. Cohort 1 (n = 44): Female patients without a cancer diagnosis, aged 17–50 years (median 35 years). Surplus serum samples originally sent to the laboratory for oestradiol measurement were anonymised and stored at −20°C before analysis. This group was used to establish the expected comparison between each immunoassay and LC-MS/MS in a general population.
2. Cohort 2 (n = 16): Post-menopausal women with a breast cancer diagnosis who were not receiving endocrine therapy. A total of 17 samples were received. Their median age was 50 years (range 39–59). This group served as the control population.
3. Cohort 3 (n = 10): Women with advanced breast cancer who were actively receiving treatment with fulvestrant at the time of sampling.

All statistical analyses were performed using Analyse-it software for Microsoft Excel.

Key Findings: What the Tests Revealed

Results in Women Without Cancer (Cohort 1)

In the 44 women without cancer, the results from both immunoassays correlated very closely with those from LC-MS/MS. The Pearson correlation coefficient (R²) was 0.98 for Siemens versus LC-MS/MS and 0.99 for Abbott versus LC-MS/MS — indicating strong overall agreement in this general population.

However, closer analysis using Bland-Altman analysis, which examines how well two methods agree across the range of values, revealed important systematic differences. The Abbott test showed a negative bias of −18.6 pmol/L (95% limits of agreement: −85.8 to 48.5), meaning it tended to read lower than the gold standard. The Siemens test showed a positive bias of +37.9 pmol/L (95% limits of agreement: −27.7 to 103.5), meaning it tended to read higher. In plain terms, both immunoassays inaccurately estimated oestradiol levels at the low concentrations that matter most for determining menopausal status.

Results in Post-Menopausal Breast Cancer Patients Not on Treatment (Cohort 2)

In the control group of post-menopausal breast cancer patients, 41% of samples had detectable oestradiol levels (above the test-specific detection limit) when measured by LC-MS/MS. But the immunoassays performed far worse: only 12% of samples showed detectable levels with either Siemens or Abbott. This demonstrates how insensitive immunoassays are in the very low oestradiol range typical of post-menopausal women.

Results in Women Taking Fulvestrant (Cohort 3)

The findings in the 10 women receiving fulvestrant were striking and concerning. Oestradiol levels fell into the pre-menopausal range for:

• 10 out of 10 women (100%) by the Siemens test
• 8 out of 10 women (80%) by the Abbott test
• 0 out of 10 women (0%) by LC-MS/MS

The median oestradiol levels were dramatically different depending on the test used:

• Siemens: 356 pmol/L (range 191–761)
• Abbott: 186 pmol/L (range <92–263)
• LC-MS/MS: 14 pmol/L (range <10–43)

The differences between each immunoassay and LC-MS/MS were highly statistically significant (Mann-Whitney p < 0.001), meaning there is less than a 0.1% chance these results occurred by random chance. To put these numbers in context, the upper limit of the post-menopausal oestradiol range for each assay was highlighted in the study: 118 pmol/L for Siemens, 103 pmol/L for Abbott, and 77 pmol/L for the mass spectrometry assay. By the Siemens test, every single woman on fulvestrant appeared to have oestradiol levels more than three times above the post-menopausal threshold.

Persistent Elevation After Treatment Stops

Perhaps even more concerning was the finding in two additional patients whose oestradiol levels were measured 2 and 4 months after their final dose of fulvestrant. Even at these later time points, the test results remained falsely elevated:

• At 2 months: Siemens read 165 pmol/L, Abbott read <92 pmol/L, but LC-MS/MS showed just 20 pmol/L
• At 4 months: Siemens read 283 pmol/L, Abbott read 124 pmol/L, but LC-MS/MS showed <10 pmol/L

These results illustrate that the interference can persist for months after treatment ends. This is because fulvestrant is administered as an intramuscular injection, which results in slow absorption, and the drug has a half-life of approximately 54 days at both the 250 mg and 500 mg doses. In practical terms, the drug remains in the body — and continues to interfere with tests — for a very long time.

Discussion: Understanding the Interference Problem

This study provides clear, direct evidence that fulvestrant interferes with two of the most commonly used oestradiol immunoassays in routine clinical practice. Although the issue was first raised in a 2015 case report, followed by Field Safety Notices, regulatory alerts, and label changes, the researchers note that this information has not effectively reached practising oncologists.

The slow absorption and long half-life of fulvestrant make the problem particularly dangerous. As the drug's levels decline gradually over many weeks, the window of time during which blood tests could produce misleading results is very long. The patients in this study who still showed elevated immunoassay readings 2 and 4 months after their last injection demonstrate that clinicians cannot simply wait a few weeks after treatment stops before relying on these tests.

It is also important to note that even in a normal population of women without cancer, both immunoassays overestimated oestradiol in the low concentration range. This is exactly the range that doctors rely on when making decisions about menopausal status. While immunoassays are widely available, inexpensive, and perfectly suitable for measuring oestradiol in pre-menopausal women — where levels are much higher — they are fundamentally limited when used in the breast cancer population.

Clinical Implications: What This Means for Patients

A spuriously elevated oestradiol level in a woman taking fulvestrant can have serious real-world consequences. It may lead to:

• Unnecessary investigation of ovarian reserve — testing and procedures to check egg supply that are not medically indicated
• Inappropriate ovarian function suppression — treatments to shut down ovarian function that a woman does not actually need
• Exclusion from clinical trials — being denied enrolment in potentially beneficial research studies because her "oestradiol level" makes her appear pre-menopausal, even if she is truly post-menopausal
• Delayed or incorrect treatment decisions — because endocrine therapy choices often depend on whether a woman is pre- or post-menopausal

The researchers emphasise that both immunoassays gave consistently higher results than LC-MS/MS in the oestradiol range of interest for establishing menopausal status. They conclude that immunoassays should not be used to assess menopausal status in women currently or recently treated with fulvestrant.

The MHRA has already recommended using mass spectrometry assays for oestradiol estimation in women receiving fulvestrant, and the data in this study strongly support that recommendation.

Limitations: What This Study Could Not Prove

This was a service evaluation rather than a large controlled research trial, and the patient cohorts were relatively small — particularly cohort 3, which included just 10 women taking fulvestrant. The study tested two specific immunoassays (Siemens Centaur XP and Abbott Architect); other immunoassay brands used elsewhere may behave differently, although the structural similarity between fulvestrant and oestradiol suggests the cross-reactivity problem is likely to be widespread across antibody-based tests.

The researchers also acknowledged practical challenges with the recommended alternative: mass spectrometry equipment is expensive, and oestradiol is a technically difficult substance to measure. The analysis requires specialised expertise and should only be performed in centres with validated assays and appropriate quality controls. This means that not every hospital can offer LC-MS/MS testing in-house, although in the UK samples can be sent by first-class post to specialist referral centres.

Recommendations: What Patients and Doctors Should Do

Based on the findings of this study, the authors offer clear guidance for clinical practice:

1. Do not use immunoassays to determine menopausal status in women who are currently receiving fulvestrant or who have recently stopped taking it.
2. Use LC-MS/MS instead when oestradiol measurement is needed in this patient group, as it provides the sensitivity and specificity required to accurately assess menopausal status.
3. Improve access to mass spectrometry — expanding the availability of these tests would meaningfully improve the management of patients with ER-positive breast cancer.
4. Increase clinician awareness — oncologists and laboratory staff need to be reminded that fulvestrant can interfere with oestradiol immunoassays and that the effects can persist for months after the last dose.

For patients, the take-home message is simple: if you are taking fulvestrant (or have taken it recently) and your doctor orders a blood test to check your oestrogen levels or confirm your menopausal status, ask whether the laboratory is using a mass spectrometry method rather than a standard immunoassay. Knowing which test is being used can help ensure that your treatment decisions are based on accurate information.

Frequently Asked Questions

Why can fulvestrant (Faslodex) make my estrogen blood test results falsely high?

Fulvestrant is structurally very similar to oestradiol, the main form of estrogen. Standard antibody-based immunoassays can mistake fulvestrant for oestradiol, producing falsely elevated results. In a study of 10 women taking fulvestrant, all 10 appeared to have pre-menopausal levels by one immunoassay, while a mass spectrometry test correctly showed none did.

What blood test should I ask for if I am taking fulvestrant?

Ask your doctor if the laboratory uses mass spectrometry (LC-MS/MS) rather than a standard immunoassay. Mass spectrometry is highly sensitive and specific, and it correctly showed post-menopausal estrogen levels in all 10 women taking fulvestrant, while two common immunoassays gave falsely high readings.

What are the risks of getting a falsely high estrogen test result while on fulvestrant?

A falsely high result may lead to unnecessary ovarian reserve testing, inappropriate ovarian suppression treatment, exclusion from clinical trials, or incorrect treatment decisions. Doctors might think you are pre-menopausal when you are actually post-menopausal, especially because both immunoassays gave consistently higher results than mass spectrometry in the low estrogen range.

Are standard estrogen immunoassays reliable for post-menopausal women not taking fulvestrant?

No, they are not reliable in the low estrogen range typical of post-menopausal women. In a control group of post-menopausal breast cancer patients not on endocrine therapy, only 12% of samples showed detectable estrogen by either immunoassay, while mass spectrometry detected estrogen in 41%. Both immunoassays also overestimated estrogen in low concentrations in women without cancer.

What should I do if my doctor orders an estrogen test while I am on fulvestrant?

Ask specifically whether the laboratory will use mass spectrometry (LC-MS/MS) instead of a standard immunoassay. If only immunoassay is available, ask if your sample can be sent to a specialist referral centre. Knowing which test is used can help ensure your menopausal status and treatment decisions are based on accurate information.

Did the study prove that all fulvestrant-treated women get false results with every immunoassay?

No. The study tested two specific immunoassays—Siemens Centaur XP and Abbott Architect—in just 10 women taking fulvestrant. All 10 appeared pre-menopausal by Siemens, and 8 of 10 by Abbott. Other brands may behave differently, but the structural similarity of fulvestrant and oestradiol suggests the problem is likely widespread.

If I take fulvestrant and my estrogen blood test shows pre-menopausal levels, should I get a second opinion?

Fulvestrant can interfere with antibody-based estradiol tests, causing falsely high levels. One evaluation showed that all 10 women on fulvestrant appeared pre-menopausal by one immunoassay and 8 of 10 by another, while mass spectrometry correctly identified none as pre-menopausal. This interference can persist for months after the last dose. If your estradiol was measured by an immunoassay while you take or recently took fulvestrant, a second opinion can help ensure your treatment decisions are based on accurate test results. Diagnostic Detectives Network provides independent expert second opinions.

Source Information

Original Article: "Oestradiol measurement during fulvestrant treatment for breast cancer"

Authors: Laura J. Owen, Phillip J. Monaghan, Anne Armstrong, Brian G. Keevil, Claire Higham, Zena Salih, and Sacha Howell

Journal: British Journal of Cancer (2019), Volume 120, pages 404–406

DOI: https://doi.org/10.1038/s41416-019-0378-9

Published: Online 25 January 2019

Affiliation: Departments of Biochemistry, Medical Oncology, and Endocrinology at Wythenshawe Hospital and The Christie, Manchester, UK

This patient-friendly article is based on peer-reviewed research.