Laboratory Testing for Pregnancy
Order a serum or urine human chorionic gonadotropin (hCG) test to check for pregnancy. 1
Recommended Testing Approach
Primary Test Options
You have two equally valid options for pregnancy testing:
- Serum hCG (quantitative or qualitative): Measures hCG in blood
- Urine hCG: Measures hCG in urine using immunoassay
A patient is considered pregnant when serum hCG is greater than 5 mIU/mL 1. This threshold is the established standard, though values of 5 mIU/mL or less may occur in healthy non-pregnant patients.
Quantitative vs. Qualitative Serum Testing
While both quantitative and qualitative serum hCG tests have similar performance characteristics (negative predictive value 99.9% for both), quantitative hCG testing provides more clinical utility 2. The quantitative test gives you:
- A specific numeric value for clinical decision-making
- Ability to track serial values for pregnancy viability assessment
- Correlation with ultrasound findings (gestational sac typically visible on transvaginal ultrasound when hCG reaches 1,000-3,000 mIU/mL) 3
Qualitative serum testing could be considered obsolete given that quantitative tests provide the same diagnostic accuracy with additional clinical information 2.
Urine Testing Performance
Urine pregnancy tests using modern monoclonal antibodies to beta-hCG are highly sensitive and specific:
- Sensitivity of 25 IU/L with most current kits 4
- Positive results possible 3-4 days after implantation
- 98% positive by 7 days after implantation (time of expected period) 4
- A negative result 1 week after a missed period virtually guarantees the woman is not pregnant 4
Point-of-Care Considerations
Whole blood can be substituted for urine in bedside pregnancy testing with similar or greater accuracy (95.8% sensitivity for whole blood vs. 95.3% for urine, both with 100% specificity) 5. This is useful when urine samples are difficult to obtain.
Important Clinical Caveats
Laboratory Variability in Reporting
Be aware that significant variation exists in how laboratories report hCG results 6:
- Only 42% of laboratories use the well-established 5.0 IU/L cutoff
- 14% use 25.0 IU/L as their threshold
- 68% provide interpretations like "positive" or "negative"
Verify your laboratory's specific cutoff values and reporting practices to avoid misinterpretation.
Timing Considerations
- hCG first appears in maternal blood 6-8 days following fertilization 4
- Levels rise rapidly, peaking at 7-10 weeks gestation
- For reliable results, testing should ordinarily be delayed until the time of the missed menses 7
False Results
False-positive results due to interfering materials are extremely unlikely with current beta-subunit specific tests 4. However, be aware that:
- Rarely, elevated hCG may be unrelated to pregnancy (pituitary dysfunction, neoplasia) 1
- Different laboratories use different hCG assays with varying terminology (some no longer use the "beta" modifier) 1
What NOT to Order
Do not order urine glucose testing for pregnancy detection - it is not recommended for routine care 8. Urine glucose testing has no role in pregnancy diagnosis.