A 34-year-old woman has persistently low serum beta-hCG values around 70 mIU/mL for 5 months following a first-trimester miscarriage. Imaging shows no retained tissue, no uterine lesion, and no metastases. Her urine pregnancy test is NEGATIVE despite the serum value. Serial dilution of her serum gives non-proportional results. What explains these findings?

• A Placental site trophoblastic tumour producing free beta subunit
• B Quiescent gestational trophoblastic disease
• C Heterophile antibody interference in the immunoassay ✓
• D Exogenous hCG administration for ovulation induction

Explanation

Heterophile antibodies bridge the capture and detection antibodies in sandwich assays, generating a falsely positive signal. Because these large antibody complexes do not cross the glomerular filter, the urine test is negative, which is the bedside clue. Non-proportional behaviour on serial dilution confirms assay interference, and the absence of any trophoblastic tumour excludes malignancy. Recognising phantom hCG prevents unnecessary chemotherapy and imaging.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.