Endocrine Abstracts

Introduction: Fulvestrant, a selective oestrogen receptor regulator and Exemestane, a steroidal aromatase inhibitor therapy, are used in the treatment of oestrogen receptor positive breast cancer. Most manufacturers of oestradiol (E2) immunoassays state that Fulvestrant interferes in their assay, some also state that aromatase inhibitors may interfere.

Method: Sample pools (n=3) were prepared from off-the-clot human serum obtained from female donors: Sample A – the base serum with no added analytes, Sample B – the base pool with Fulvestrant (25ng/mL) added, and Sample C – the base pool with Exemestane (150pg/mL) added. The samples were distributed to all participants measuring E2 in the UK NEQAS for Steroid Hormones EQA Scheme (Distribution 523). The cross-reactivity of each method was calculated for both drugs, comparing the results obtained from the base pool with those from the spiked samples. Participants were asked to answer some web Q&A’s on how requests are processed for patients on Fulvestrant and Exemestane.

Results: No significant changes in E2 concentrations were observed in samples containing Exemestane when compared to the base pool. However, in samples containing Fulvestrant a positive bias was observed for several methods including the Siemens ADVIA Centaur and Atellica, and Abbott Alinity and Architect methods. Negligible cross-reactivity was observed in other manufacturers’ methods and the LC-MS/MS group. Of the Q&A respondents, 85% do not vet oestradiol requests on the basis of clinical details, only 27% of respondents refer E2 requests for analysis by LC-MS/MS at the request of the clinician or after reviewing clinical details.

Conclusion: Fulvestrant significantly interferes in some E2 immunoassays and may lead to over-estimation of oestradiol concentrations in patients receiving therapy. While most manufacturers state that Fulvestrant interferes in oestradiol immunoassays, many laboratories do not act on this information. This could lead to inappropriate management of patients being treated with Fulvestrant.