Endocrine Abstracts

JOINT1451

Introduction: Fulvestrant is a selective oestrogen receptor regulator and Exemestane is a steroidal aromatase inhibitor therapy, both 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 (25 ng/mL) added, and Sample C – the base pool with Exemestane (150 pg/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 150 pg/mL Exemestane when compared to the base pool. However, in samples containing 25 ng/mL Fulvestrant a positive bias was observed for several methods. A large positive bias was observed in the Siemens ADVIA Centaur and Siemens Atellica methods. Cross-reactivity was also observed in the Abbott Alinity and Abbott Architect methods but to a lesser extent. Negligible cross-reactivity was observed in other immunoassay methods (Beckman Access/DxI, Roche Cobas, Roche Cobas Pro, QuidelOrtho) and the LC-MS/MS group. The web Q&A’s received 158 responses (response rate 56%). Of the respondents, 85% analysed all E2 requests and did not vet on the basis of clinical details. 27% of respondents refer E2 requests for analysis by LC-MS/MS at the request of the clinician or after reviewing clinical details. 59% of respondents did not append comments to E2 results in regard to potential assay interference from medications including Fulvestrant.

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