Heterophile antibody blocking agent to confirm false positive serum human chorionic gonadotropin assay

BACKGROUND: Persistently false positive human chorionic gonadotropin (hCG) test results in the absence of an intrauterine pregnancy may lead to a diagnosis of ectopic pregnancy or gestational trophoblastic neoplasia and unnecessary testing and therapy. CASES: A 38-year-old woman on oral contraceptives and a 53-year-old perimenopausal woman presented with persistently elevated serum beta-hCG levels from 30 to 225 mIU/ mL. Both women had a history of working with mice. Urine hCG testing was negative. Serum beta-hCG levels for both patients were negative after pretreatment of their serum with a heterophile antibody blocking agent. CONCLUSION: These cases illustrate that serum heterophile antibodies can interfere with the hCG enzyme immunoassay and result in false positive values. The addition of heterophile blocking agent to the serum can exclude false positives, thereby preventing unnecessary evaluation and treatment.