Curious phenomenon of 'hook effect' in invasive mole

Gestational trophoblastic disease is a group of pregnancy-related trophoblastic tumours. Gestational trophoblastic neoplasia refers to its invasive and malignant forms: invasive mole, choriocarcinoma, placental site trophoblastic tumour and epithelioid trophoblastic tumour. Patients exhibit elevated beta human chorionic gonadotropin (β hCG) levels, sometimes exceeding 100 000 mIU/mL. When the serum levels of hCG surpass 500 000 mIU/mL, there is a possibility of encountering a phenomenon called 'hook effect' which causes erroneously low or negative value when using the immunometric hCG assays that are presently available in the commercial market. We present a rare case involving a female in mid-30s with an invasive molar pregnancy presenting with bleeding, an abdominal mass, hypertension and hyperthyroidism. Despite a negative urine β hCG test, her serum hCG, after dilution, was recalculated to over 850 000 mIU/mL. MRI confirmed invasive mole. Post-hysterectomy, she received methotrexate chemotherapy. Her β hCG normalised after two cycles, and she remains disease-free for 14 months.