PS5-06-14: Unexpected B-hCG elevation in metastatic inflammatory breast cancer: A diagnostic dilemma

Abstract Introduction: The beta subunit of human chorionic gonadotropin (B-hCG) is often used as a marker of pregnancy, but can be produced by several different sources, including the placenta, trophoblastic tumors, nontrophoblastic tumors, and the pituitary gland. While not used as a routine tumor marker for breast malignancies, studies have shown that hCG from a placental source appears to be protective against breast cancer, while ectopic production of B-hCG appears to be associated with poor prognosis. As part of routine testing during chemotherapy at our institution, all patients undergo a pregnancy test due to the risks of chemotherapy on the developing fetus. Here, we present a case of incidentally found false positive pregnancy test in a post-menopausal patient with metastatic inflammatory breast cancer. Case Description: We present a case of a 49 year old female who was initially diagnosed in May 2024 with stage IV HER2+ inflammatory breast cancer, with metastases to multiple lymph nodes and pulmonary nodules. She progressed through several lines of therapy, including docetaxel, trastuzumab, and pertuzumab, dose dense doxorubicin and cyclophosphamide, and fam-trastuzumab deruxtecan. She is currently on therapy with tucatinib, capecitabine, and trastuzumab. In May 2025, she had routine prechemotherapy labs done including a urine pregnancy test which resulted positive. She had two prior urine pregnancy tests which were negative in July and September 2024. We subsequently performed a serum B-hCG quantitative test in May 2025 showed a B-hCG of 82.4 mIU/mL. The patient had reported abstinence and amenorrhea for the past year. It was determined to be a false pregnancy test and her therapy was continued. The clinical case was discussed with our OB/GYN colleagues with consensus to verify menopausal status by hormone levels. Labwork showed an estradiol of