Pregnancy Detection: Routine Blood Work vs. Targeted hCG Testing

Routine blood panels (CBC, CMP, lipid panel, TSH) do not detect pregnancy. Pregnancy diagnosis requires specific measurement of human chorionic gonadotropin (hCG).

hCG Testing Modalities

Test	Sample	Detection Threshold	Time to Positive	Use
Qualitative urine hCG	Urine	20-25 mIU/mL	~14 days post-ovulation (day of missed period)	Screening, home use
Quantitative serum hCG	Blood	1-5 mIU/mL	~8-10 days post-ovulation (pre-missed period)	Confirmation, monitoring, ectopic evaluation
Qualitative serum hCG	Blood	5-10 mIU/mL	~10-12 days post-ovulation	Rapid hospital confirmation

Why Routine Blood Work Misses Pregnancy

• hCG is not part of standard panels (CBC, BMP/CMP, LFTs, lipids, TSH, etc.).
• Pregnancy causes secondary changes (↑WBC, ↓creatinine, ↑alk phos, ↓TSH 1st tri) but these are non-specific.
• Providers must order hCG explicitly.

Clinical Indications for hCG Testing

• Missed period + sexual activity
• Abdominal pain/vaginal bleeding (rule out ectopic)
• Pre-procedure (surgery, radiation, teratogenic drugs)
• Monitoring: viability (doubling q48h), ectopic (suboptimal rise), molar pregnancy (very high), post-miscarriage/abortion (decline to zero).

Interpretation Pearls

• Discriminatory zone: hCG 1500-2000 mIU/mL → should see IUP on transvaginal US.
• hCG doubling time: 48h (early), slows as levels rise; plateau → evaluate for non-viability/ectopic.
• Hook effect: Very high hCG (>500,000) → false low on some assays; dilute and repeat.

If pregnancy is possible, request hCG specifically. "Routine blood work" is not a pregnancy test.