Lab Urine Pregnancy Test Accuracy: False Positives and Negatives

Lab urine hCG tests are >99% sensitive/specific when used correctly, but errors occur:

False Positives (Rare)

• hCG-containing medications: Fertility drugs (Ovidrel, Pregnyl, Profasi) — can persist 7–14 days after injection.
• Medical conditions: Trophoblastic disease (molar pregnancy, choriocarcinoma), pituitary hCG (perimenopausal), some cancers (ovarian, bladder, lung).
• Heterophile antibodies interfering with assay.
• Chemical pregnancy: Very early loss after implantation; hCG briefly detectable.

False Negatives

• Testing too early: Before hCG reaches threshold (~20–25 mIU/mL). Wait until missed period.
• Dilute urine: Excessive fluid intake → low concentration. Use first-morning urine.
• Hook effect: Extremely high hCG (>500,000 mIU/mL) saturates antibodies → false low. Dilute specimen to resolve.
• Faulty test/expired kit.

Confirmation

Positive urine test → quantitative serum β-hCG (more sensitive, gives exact level). Serial levels: should double every 48 hrs in viable early pregnancy. Ultrasound at ~5–6 weeks confirms intrauterine gestation.