HCG as a Pregnancy Test
Yes, human chorionic gonadotropin (HCG) is the standard marker used in all pregnancy tests, both professional laboratory assays and home-based tests. 1, 2
Why HCG is the Marker of Choice
HCG is the ideal marker for pregnancy detection because it rises rapidly and consistently in early pregnancy and can be reliably detected in both blood and urine. 2, 3 No other pregnancy-specific material can match the sensitivity and convenience offered by HCG immunoassay, and even ultrasound detection is not as sensitive as HCG measurement for early pregnancy detection. 2
Detection Timeline and Sensitivity
HCG becomes detectable in maternal blood approximately 6-9 days after conception (fertilization), with serum levels initially rising above 5 mIU/mL to confirm pregnancy. 4, 2
Urine pregnancy tests can detect HCG 3-4 days after implantation with current test kits that have a sensitivity of 25 units per liter. 2
By 7 days after the missed period, 98% of pregnancy tests will be positive if the woman is pregnant. 2
A negative result 1 week after a missed period virtually guarantees the woman is not pregnant. 2
Types of HCG Tests Available
Qualitative Tests
Urine pregnancy tests (both home and office-based) detect HCG at concentrations of 20-25 mIU/mL, though they may not detect very early pregnancies or may remain positive for several weeks after pregnancy termination. 1
Most qualitative pregnancy tests require an additional 11 days past the expected menses to detect 100% of pregnancies. 1
Quantitative Tests
Serum HCG testing provides more accurate results, particularly when the timing of conception is uncertain or when monitoring pregnancy progression is needed. 1
Serial quantitative HCG measurements every 48 hours are essential for distinguishing between viable pregnancies, miscarriages, and ectopic pregnancies when ultrasound findings are indeterminate. 1
Clinical Applications Beyond Basic Pregnancy Detection
HCG testing serves multiple critical clinical purposes beyond simple pregnancy confirmation:
Prenatal screening for fetal aneuploidy: HCG (or free beta-HCG) is measured along with PAPP-A and nuchal translucency between 11-14 weeks gestation for Down syndrome screening, achieving detection rates of 82-86% at a 5% false-positive rate. 1, 5
Monitoring gestational trophoblastic disease: Serum HCG monitoring at least once every 2 weeks until normalization is required after molar pregnancy diagnosis. 1
Diagnosing ectopic pregnancy: The discriminatory threshold of approximately 3,000 mIU/mL (the level at which a gestational sac should be visible on transvaginal ultrasound) helps identify ectopic pregnancies when no intrauterine pregnancy is visualized. 1, 4
Tumor marker: HCG serves as a marker for gestational trophoblastic disease, choriocarcinoma, and testicular carcinoma. 6, 7
Important Technical Considerations
Current pregnancy tests are based on monoclonal antibodies to the beta-subunit of HCG, which virtually eliminate cross-reaction with pituitary luteinizing hormone (LH) and permit high sensitivity. 2 However, practitioners should be aware that:
Different HCG assays may have varying sensitivities and specificities due to their ability to detect different HCG isoforms and fragments. 1, 7
When HCG results do not fit the clinical picture, measure HCG on a different assay, as different assays detect different forms of HCG. 1
Using the same laboratory for serial measurements is recommended to ensure consistency. 1
Common Pitfalls
False-positive results can occur due to assay-interfering molecules, though with current generation test kits these are extremely unlikely. 1, 2
False-negative results can occur with very early pregnancy when HCG levels are still low, or if the assay doesn't detect the specific forms of HCG present. 1, 8
HCG can remain detectable for several weeks after pregnancy termination (spontaneous or induced), potentially causing confusion. 1
When urine and serum results are discrepant, the serum result is generally more reliable, though cross-reactive molecules causing false-positive serum results rarely appear in urine. 1