{"product_id":"how-fulvestrant-treatment-can-skew-estrogen-blood-test-results-what-breast-cancer-patients-need-to-know","title":"How Fulvestrant Treatment Can Skew Estrogen Blood Test Results: What Breast Cancer Patients Need to Know","description":"\u003cp\u003eResearchers in the United Kingdom have confirmed that fulvestrant (Faslodex) — a common breast cancer drug — can interfere with two widely used estrogen (oestradiol) blood tests, producing falsely elevated results that could lead to incorrect treatment decisions and unfair exclusion from clinical trials. In a study of 10 women taking fulvestrant, all 10 appeared to have pre-menopausal oestradiol levels by one immunoassay and 8 of 10 by another, while a highly sensitive mass spectrometry method correctly showed none of them did. The researchers strongly recommend that mass spectrometry (LC-MS\/MS) replace standard immunoassays for measuring oestradiol in women currently or recently treated with fulvestrant.\u003c\/p\u003e\n\n\u003ch1\u003eHow Fulvestrant Treatment Can Skew Estrogen Blood Test Results: What Breast Cancer Patients Need to Know\u003c\/h1\u003e\n\n\u003ch2\u003eTable of Contents\u003c\/h2\u003e\n\u003cul\u003e\n\u003cli\u003e\u003ca href=\"#ddn-key-points\"\u003eKey Points\u003c\/a\u003e\u003c\/li\u003e\n\n  \u003cli\u003e\u003ca href=\"#background\"\u003eBackground: Why Accurate Estrogen Testing Matters\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#methods\"\u003eStudy Methods: How the Research Was Conducted\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#findings\"\u003eKey Findings: What the Tests Revealed\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#discussion\"\u003eDiscussion: Understanding the Interference Problem\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#implications\"\u003eClinical Implications: What This Means for Patients\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#limitations\"\u003eLimitations: What This Study Could Not Prove\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#recommendations\"\u003eRecommendations: What Patients and Doctors Should Do\u003c\/a\u003e\u003c\/li\u003e\n  \u003cli\u003e\u003ca href=\"#ddn-faq\"\u003eFrequently Asked Questions\u003c\/a\u003e\u003c\/li\u003e\n\u003cli\u003e\u003ca href=\"#source\"\u003eSource Information\u003c\/a\u003e\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003c!-- ddn:keypoints:start --\u003e\n\u003ch2 id=\"ddn-key-points\"\u003eKey Points\u003c\/h2\u003e\n\u003cul\u003e\n\u003cli\u003eIn a study of 10 women taking fulvestrant, two common immunoassays falsely showed pre-menopausal estrogen levels in 100% and 80% of patients.\u003c\/li\u003e\n\u003cli\u003eMass spectrometry (LC-MS\/MS) correctly showed all 10 women on fulvestrant had post-menopausal estrogen levels, with a detection limit far lower than immunoassays.\u003c\/li\u003e\n\u003cli\u003eFulvestrant interference can persist for months after the last dose; two patients had falsely elevated readings 2 and 4 months later.\u003c\/li\u003e\n\u003cli\u003eStandard immunoassays are not sensitive enough for post-menopausal estrogen levels, overestimating low concentrations even in women without cancer.\u003c\/li\u003e\n\u003cli\u003eResearchers recommend using LC-MS\/MS instead of immunoassays to assess menopausal status in women currently or recently treated with fulvestrant.\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c!-- ddn:keypoints:end --\u003e\n\n\n\u003ch2 id=\"background\"\u003eBackground: Why Accurate Estrogen Testing Matters\u003c\/h2\u003e\n\n\u003cp\u003eDetermining whether a woman is pre- or post-menopausal is a critical step in planning treatment for \u003cstrong\u003eoestrogen receptor (ER)-positive breast cancer\u003c\/strong\u003e — 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 \u003cstrong\u003eoestradiol\u003c\/strong\u003e (the main form of oestrogen) in the blood.\u003c\/p\u003e\n\n\u003cp\u003eMost hospital laboratories use \u003cstrong\u003eimmunoassays (IAs)\u003c\/strong\u003e — 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.\u003c\/p\u003e\n\n\u003cp\u003eAn additional and more alarming issue emerged in 2016. Manufacturers of commercial immunoassays released \u003cstrong\u003eField Safety Notices\u003c\/strong\u003e describing a significant cross-reactivity problem with \u003cstrong\u003efulvestrant\u003c\/strong\u003e (brand name Faslodex), a drug used to treat advanced ER-positive breast cancer. Fulvestrant is a \u003cstrong\u003eselective oestrogen receptor degrader\u003c\/strong\u003e — it works by binding to oestrogen receptors on cancer cells and causing the receptors to be destroyed.\u003c\/p\u003e\n\n\u003cp\u003eBecause 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 \u003cstrong\u003eFood and Drug Administration (FDA)\u003c\/strong\u003e and the UK's \u003cstrong\u003eMedicines and Healthcare products Regulatory Agency (MHRA)\u003c\/strong\u003e — 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.\"\u003c\/p\u003e\n\n\u003cp\u003eDespite 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.\u003c\/p\u003e\n\n\u003ch2 id=\"methods\"\u003eStudy Methods: How the Research Was Conducted\u003c\/h2\u003e\n\n\u003cp\u003eTo investigate the extent of the problem, the team conducted a \u003cstrong\u003eservice evaluation\u003c\/strong\u003e — 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.\"\u003c\/p\u003e\n\n\u003ch3\u003eThe Three Tests Compared\u003c\/h3\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eSiemens Centaur XP (SC):\u003c\/strong\u003e A commonly used immunoassay with a lower limit of quantitation (LLOQ — the lowest concentration it can accurately measure) of \u003cstrong\u003e70 pmol\/L\u003c\/strong\u003e.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eAbbott Architect (AA):\u003c\/strong\u003e Another widely used immunoassay with an LLOQ of \u003cstrong\u003e92 pmol\/L\u003c\/strong\u003e.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eLC-MS\/MS (liquid chromatography-tandem mass spectrometry):\u003c\/strong\u003e A highly sensitive and specific analytical method with an LLOQ of just \u003cstrong\u003e10 pmol\/L\u003c\/strong\u003e, meaning it can detect far lower levels of oestradiol than the immunoassays.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eThe 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 \u003cstrong\u003ecoefficients of variation (CVs)\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003ch3\u003eThe Patient Groups Studied\u003c\/h3\u003e\n\n\u003cp\u003eThree groups of women were included in the evaluation:\u003c\/p\u003e\n\n\u003col\u003e\n  \u003cli\u003e\n\u003cstrong\u003eCohort 1 (n = 44):\u003c\/strong\u003e Female patients \u003cem\u003ewithout\u003c\/em\u003e 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.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eCohort 2 (n = 16):\u003c\/strong\u003e Post-menopausal women with a breast cancer diagnosis who were \u003cem\u003enot\u003c\/em\u003e receiving endocrine therapy. A total of 17 samples were received. Their median age was 50 years (range 39–59). This group served as the \u003cstrong\u003econtrol population\u003c\/strong\u003e.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eCohort 3 (n = 10):\u003c\/strong\u003e Women with advanced breast cancer who were actively receiving treatment with \u003cem\u003efulvestrant\u003c\/em\u003e at the time of sampling.\u003c\/li\u003e\n\u003c\/ol\u003e\n\n\u003cp\u003eAll statistical analyses were performed using Analyse-it software for Microsoft Excel.\u003c\/p\u003e\n\n\u003ch2 id=\"findings\"\u003eKey Findings: What the Tests Revealed\u003c\/h2\u003e\n\n\u003ch3\u003eResults in Women Without Cancer (Cohort 1)\u003c\/h3\u003e\n\n\u003cp\u003eIn 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 \u003cstrong\u003e0.98\u003c\/strong\u003e for Siemens versus LC-MS\/MS and \u003cstrong\u003e0.99\u003c\/strong\u003e for Abbott versus LC-MS\/MS — indicating strong overall agreement in this general population.\u003c\/p\u003e\n\n\u003cp\u003eHowever, closer analysis using \u003cstrong\u003eBland-Altman analysis\u003c\/strong\u003e, which examines how well two methods agree across the range of values, revealed important systematic differences. The Abbott test showed a \u003cstrong\u003enegative bias of −18.6 pmol\/L\u003c\/strong\u003e (95% limits of agreement: −85.8 to 48.5), meaning it tended to read lower than the gold standard. The Siemens test showed a \u003cstrong\u003epositive bias of +37.9 pmol\/L\u003c\/strong\u003e (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.\u003c\/p\u003e\n\n\u003ch3\u003eResults in Post-Menopausal Breast Cancer Patients Not on Treatment (Cohort 2)\u003c\/h3\u003e\n\n\u003cp\u003eIn the control group of post-menopausal breast cancer patients, \u003cstrong\u003e41% of samples had detectable oestradiol levels\u003c\/strong\u003e (above the test-specific detection limit) when measured by LC-MS\/MS. But the immunoassays performed far worse: only \u003cstrong\u003e12% of samples\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003ch3\u003eResults in Women Taking Fulvestrant (Cohort 3)\u003c\/h3\u003e\n\n\u003cp\u003eThe findings in the 10 women receiving fulvestrant were striking and concerning. Oestradiol levels fell into the \u003cstrong\u003epre-menopausal range\u003c\/strong\u003e for:\u003c\/p\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003e10 out of 10\u003c\/strong\u003e women (100%) by the Siemens test\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003e8 out of 10\u003c\/strong\u003e women (80%) by the Abbott test\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003e0 out of 10\u003c\/strong\u003e women (0%) by LC-MS\/MS\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eThe median oestradiol levels were dramatically different depending on the test used:\u003c\/p\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eSiemens:\u003c\/strong\u003e 356 pmol\/L (range 191–761)\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eAbbott:\u003c\/strong\u003e 186 pmol\/L (range \u0026lt;92–263)\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eLC-MS\/MS:\u003c\/strong\u003e 14 pmol\/L (range \u0026lt;10–43)\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eThe differences between each immunoassay and LC-MS\/MS were highly statistically significant (\u003cstrong\u003eMann-Whitney p \u0026lt; 0.001\u003c\/strong\u003e), 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: \u003cstrong\u003e118 pmol\/L for Siemens\u003c\/strong\u003e, \u003cstrong\u003e103 pmol\/L for Abbott\u003c\/strong\u003e, and \u003cstrong\u003e77 pmol\/L for the mass spectrometry assay\u003c\/strong\u003e. By the Siemens test, every single woman on fulvestrant appeared to have oestradiol levels more than three times above the post-menopausal threshold.\u003c\/p\u003e\n\n\u003ch3\u003ePersistent Elevation After Treatment Stops\u003c\/h3\u003e\n\n\u003cp\u003ePerhaps even more concerning was the finding in two additional patients whose oestradiol levels were measured \u003cstrong\u003e2 and 4 months after their final dose of fulvestrant\u003c\/strong\u003e. Even at these later time points, the test results remained falsely elevated:\u003c\/p\u003e\n\n\u003cul\u003e\n  \u003cli\u003eAt 2 months: Siemens read \u003cstrong\u003e165 pmol\/L\u003c\/strong\u003e, Abbott read \u003cstrong\u003e\u0026lt;92 pmol\/L\u003c\/strong\u003e, but LC-MS\/MS showed just \u003cstrong\u003e20 pmol\/L\u003c\/strong\u003e\n\u003c\/li\u003e\n  \u003cli\u003eAt 4 months: Siemens read \u003cstrong\u003e283 pmol\/L\u003c\/strong\u003e, Abbott read \u003cstrong\u003e124 pmol\/L\u003c\/strong\u003e, but LC-MS\/MS showed \u003cstrong\u003e\u0026lt;10 pmol\/L\u003c\/strong\u003e\n\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eThese 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 \u003cstrong\u003ehalf-life of approximately 54 days\u003c\/strong\u003e 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.\u003c\/p\u003e\n\n\u003ch2 id=\"discussion\"\u003eDiscussion: Understanding the Interference Problem\u003c\/h2\u003e\n\n\u003cp\u003eThis 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.\u003c\/p\u003e\n\n\u003cp\u003eThe 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.\u003c\/p\u003e\n\n\u003cp\u003eIt 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.\u003c\/p\u003e\n\n\u003ch2 id=\"implications\"\u003eClinical Implications: What This Means for Patients\u003c\/h2\u003e\n\n\u003cp\u003eA spuriously elevated oestradiol level in a woman taking fulvestrant can have serious real-world consequences. It may lead to:\u003c\/p\u003e\n\n\u003cul\u003e\n  \u003cli\u003e\n\u003cstrong\u003eUnnecessary investigation of ovarian reserve\u003c\/strong\u003e — testing and procedures to check egg supply that are not medically indicated\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eInappropriate ovarian function suppression\u003c\/strong\u003e — treatments to shut down ovarian function that a woman does not actually need\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eExclusion from clinical trials\u003c\/strong\u003e — being denied enrolment in potentially beneficial research studies because her \"oestradiol level\" makes her appear pre-menopausal, even if she is truly post-menopausal\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eDelayed or incorrect treatment decisions\u003c\/strong\u003e — because endocrine therapy choices often depend on whether a woman is pre- or post-menopausal\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003cp\u003eThe 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 \u003cstrong\u003eshould not be used\u003c\/strong\u003e to assess menopausal status in women currently or recently treated with fulvestrant.\u003c\/p\u003e\n\n\u003cp\u003eThe 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.\u003c\/p\u003e\n\n\u003ch2 id=\"limitations\"\u003eLimitations: What This Study Could Not Prove\u003c\/h2\u003e\n\n\u003cp\u003eThis 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.\u003c\/p\u003e\n\n\u003cp\u003eThe 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.\u003c\/p\u003e\n\n\u003ch2 id=\"recommendations\"\u003eRecommendations: What Patients and Doctors Should Do\u003c\/h2\u003e\n\n\u003cp\u003eBased on the findings of this study, the authors offer clear guidance for clinical practice:\u003c\/p\u003e\n\n\u003col\u003e\n  \u003cli\u003e\n\u003cstrong\u003eDo not use immunoassays to determine menopausal status\u003c\/strong\u003e in women who are currently receiving fulvestrant or who have recently stopped taking it.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eUse LC-MS\/MS instead\u003c\/strong\u003e when oestradiol measurement is needed in this patient group, as it provides the sensitivity and specificity required to accurately assess menopausal status.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eImprove access to mass spectrometry\u003c\/strong\u003e — expanding the availability of these tests would meaningfully improve the management of patients with ER-positive breast cancer.\u003c\/li\u003e\n  \u003cli\u003e\n\u003cstrong\u003eIncrease clinician awareness\u003c\/strong\u003e — 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.\u003c\/li\u003e\n\u003c\/ol\u003e\n\n\u003cp\u003eFor 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.\u003c\/p\u003e\n\n\u003c!-- ddn:faq:start --\u003e\n\u003ch2 id=\"ddn-faq\"\u003eFrequently Asked Questions\u003c\/h2\u003e\n\u003ch3\u003eWhy can fulvestrant (Faslodex) make my estrogen blood test results falsely high?\u003c\/h3\u003e\n\u003cp\u003eFulvestrant 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.\u003c\/p\u003e\n\u003ch3\u003eWhat blood test should I ask for if I am taking fulvestrant?\u003c\/h3\u003e\n\u003cp\u003eAsk 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.\u003c\/p\u003e\n\u003ch3\u003eWhat are the risks of getting a falsely high estrogen test result while on fulvestrant?\u003c\/h3\u003e\n\u003cp\u003eA 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.\u003c\/p\u003e\n\u003ch3\u003eAre standard estrogen immunoassays reliable for post-menopausal women not taking fulvestrant?\u003c\/h3\u003e\n\u003cp\u003eNo, 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.\u003c\/p\u003e\n\u003ch3\u003eWhat should I do if my doctor orders an estrogen test while I am on fulvestrant?\u003c\/h3\u003e\n\u003cp\u003eAsk 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.\u003c\/p\u003e\n\u003ch3\u003eDid the study prove that all fulvestrant-treated women get false results with every immunoassay?\u003c\/h3\u003e\n\u003cp\u003eNo. 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.\u003c\/p\u003e\n\u003ch3\u003eIf I take fulvestrant and my estrogen blood test shows pre-menopausal levels, should I get a second opinion?\u003c\/h3\u003e\n\u003cp\u003eFulvestrant 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.\u003c\/p\u003e\n\u003c!-- ddn:faq:end --\u003e\n\n\u003ch2 id=\"source\"\u003eSource Information\u003c\/h2\u003e\n\n\u003cp\u003e\u003cstrong\u003eOriginal Article:\u003c\/strong\u003e \"Oestradiol measurement during fulvestrant treatment for breast cancer\"\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors:\u003c\/strong\u003e Laura J. Owen, Phillip J. Monaghan, Anne Armstrong, Brian G. Keevil, Claire Higham, Zena Salih, and Sacha Howell\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eJournal:\u003c\/strong\u003e British Journal of Cancer (2019), Volume 120, pages 404–406\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eDOI:\u003c\/strong\u003e https:\/\/doi.org\/10.1038\/s41416-019-0378-9\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003ePublished:\u003c\/strong\u003e Online 25 January 2019\u003c\/p\u003e\n\u003cp\u003e\u003cstrong\u003eAffiliation:\u003c\/strong\u003e Departments of Biochemistry, Medical Oncology, and Endocrinology at Wythenshawe Hospital and The Christie, Manchester, UK\u003c\/p\u003e\n\u003cp\u003e\u003cem\u003eThis patient-friendly article is based on peer-reviewed research.\u003c\/em\u003e\u003c\/p\u003e","brand":"DiagnosticDetectives.Com","offers":[{"title":"Default Title","offer_id":47499364958364,"sku":null,"price":0.0,"currency_code":"RUB","in_stock":true}],"url":"https:\/\/diagnosticdetectives.ru\/products\/how-fulvestrant-treatment-can-skew-estrogen-blood-test-results-what-breast-cancer-patients-need-to-know","provider":"DiagnosticDetectives.Com","version":"1.0","type":"link"}