Discrepancies in estradiol levels in a premenopausal woman receiving abemaciclib despite ovarian function suppression and bilateral salpingo-oophorectomy.

Abemaciclib is approved for use in the adjuvant setting in combination with endocrine therapy for patients with high-risk, hormone receptor-positive, HER2-negative early-stage breast cancer based on the monarchE trial. Options for endocrine therapy for premenopausal women include an aromatase inhibitor with ovarian function suppression or tamoxifen with or without ovarian suppression. We describe a unique case of a premenopausal woman with early-stage breast cancer receiving adjuvant abemaciclib and an aromatase inhibitor with elevated estradiol levels as measured by the Abbott Alinity chemiluminescent immunoassay despite chemical and surgical ovarian function suppression. Given low estradiol levels using liquid chromatography-mass spectrometry testing following a bilateral salpingo-oopherectomy, our case report suggests an interference of abemaciclib with the Abbott Alinity immunoassay. This possible interference has significant impacts on clinical care as false elevations in estradiol levels measured by immunoassays can lead to unnecessary treatment changes, including surgery.