‘Hook Effect’: A Diagnostic Pitfall in Early Pregnancy

Author: Dr Rena Darbar, Dr Himanshu Gul Mirani

Institution: Midland Metropolitan University Hospital

A 30-year-old gravida 2 presented to the ED at approximately 9 weeks gestation with presyncope and severe hyperemesis (PUQE24 score 15). Despite prior positive home pregnancy tests, two ED urine pregnancy tests were negative. She was hypotensive with 4+ ketonuria. Serum hCG was markedly elevated at 222,000 mIU/mL. Bedside ultrasound confirmed a viable intrauterine pregnancy without features of molar pregnancy, which was later confirmed by formal scans. The negative urine tests were attributed to the ‘hook effect’, where extremely high hCG levels overwhelm the assay, yielding false-negative results. This case highlights the diagnostic limitations of urine pregnancy testing in the context of very high hCG concentrations. Clinicians must maintain a high index of suspicion for molar pregnancy when secondary amenorrhea, elevated hCG, and hyperemesis are present, even with negative urine tests.