What is the principle and value of the Human Chorionic Gonadotropin (HCG) test?

HCG Test: Principle and Clinical Value

Principle of HCG Testing

HCG is a glycoprotein hormone composed of an alpha subunit (common to all glycoprotein hormones like LH and TSH) and a specific beta subunit, which is why assays use antibodies directed against the beta subunit to ensure specificity. 1

Molecular Forms and Detection Challenges

• HCG exists in multiple molecular forms in biological fluids, including intact heterodimeric hormone, nicked HCG, free beta subunit (hCGβ), free alpha subunit, beta-core fragment (predominantly in urine), and nicked free beta-subunit 2, 3
• In pregnancy, the beta subunit is usually intact and becomes hyperglycosylated, particularly during the first trimester 1
• In cancer-related HCG, the beta subunit can exist in several different forms/fragments including nicked free beta, c-terminal peptide, and hyperglycosylated forms, requiring assays that can measure all forms of beta HCG equally well 1
• When excreted into urine, most HCG is broken down to the core fragment of beta-HCG (hCGβcf), which becomes the main immunoreactive form in urine during pregnancy 2

Critical Assay Limitations

• Many commercial HCG assays work well for pregnancy detection but may fail to detect all HCG isoforms/fragments in cancer patients, leading to false-negative results or significant under/over-reading of certain isoforms 1
• Several assays have particular problems with false-positive results 1
• When HCG results do not fit the clinical picture, measure HCG on a different assay, as different assays have varying sensitivities 1, 4
• When a false positive is suspected in serum, assessment of urine HCG can be helpful, as cross-reactive molecules in blood that cause false positives rarely get into urine 1, 4

Clinical Value and Applications

Pregnancy Diagnosis and Monitoring

• Serum beta-HCG becomes positive approximately 9 days after conception, making it the earliest reliable marker of pregnancy 1
• Qualitative urine pregnancy tests can detect HCG at concentrations of 20-25 mIU/mL, but may not detect very early pregnancies 4
• A negative serum beta-HCG test essentially excludes the diagnosis of intrauterine or ectopic pregnancy 1
• The discriminatory level of HCG (level at which a gestational sac should be visible on transvaginal ultrasound) is approximately 3,000 mIU/mL 4

Early Pregnancy Complications

• A single HCG measurement has limited diagnostic value; serial measurements 48 hours apart provide more meaningful clinical information 4
• In viable early intrauterine pregnancy, HCG levels typically double every 48-72 hours 4
• HCG levels that plateau (defined as <15% change over 48 hours) or rise inappropriately (<53% over 48 hours) suggest abnormal pregnancy 4
• Approximately 22% of ectopic pregnancies occur at HCG levels <1,000 mIU/mL, demonstrating that ectopic pregnancy can occur at any HCG level 4

Gestational Trophoblastic Disease

• HCG monitoring is essential after molar pregnancy treatment, with measurements every 1-2 weeks until normalization, then monthly for 6 months for complete hydatidiform mole 1, 4
• Plateauing or rising HCG levels after molar pregnancy treatment suggests development of gestational trophoblastic neoplasia (GTN) 4
• GTN is diagnosed when HCG shows a plateau for 4 consecutive values over 3 weeks, a rise >10% for 3 values over 2 weeks, or persistence 6 months or more after molar evacuation 5
• Beta-HCG levels exceeding 100,000 mIU/mL are considered a risk factor for postmolar GTN 4

Prenatal Screening for Fetal Aneuploidy

• HCG is part of first-trimester combined screening (with PAPP-A and nuchal translucency) for Down syndrome, achieving detection rates of 82-86% at 5% false-positive rate 4, 6
• In most cases of Down syndrome, HCG levels are higher than normal, while in trisomy 18, HCG levels are lower than normal 4
• Free beta-HCG performs better than intact HCG at 11 weeks (2-3% higher detection), while intact HCG may perform slightly better at 13 weeks (1-2% higher detection) 4, 6

Cancer Detection and Monitoring

• HCG is an extremely sensitive and specific marker for trophoblastic tumors of placental and germ cell origin, with treatment often initiated based on rising HCG levels alone 7
• Many nontrophoblastic tumors produce only free beta-HCG (hCGβ), which is usually a sign of aggressive disease and strongly associated with poor prognosis 7
• Elevated serum levels of hCGβ are observed in 45-60% of patients with biliary and pancreatic cancer and in 10-30% of most other cancers 7
• Specific determination of hCGβ is of value in screening for Down's syndrome and diagnosis of nontrophoblastic cancer 2

Critical Pitfalls to Avoid

False-Positive Results

• Elevated beta-HCG in a nonpregnant patient may be related to miscarriage, ectopic pregnancy, pituitary production, paraneoplastic production, or gestational trophoblastic disease 1
• Heterophilic antibodies can cause false-positive pregnancy tests 8
• Always verify discrepant results by testing with a different assay or checking urine HCG 1, 4

False-Negative Results

• False-negative pregnancy tests can occur due to delayed implantation, very early pregnancy with low HCG levels, or use of an assay that doesn't detect the specific forms of HCG present 4, 8
• Some assays fail to detect all HCG isoforms/fragments, particularly in cancer patients 1

Timing and Interpretation Errors

• Do not use HCG value alone to exclude the diagnosis of ectopic pregnancy in patients with indeterminate ultrasound findings 4
• Never initiate treatment based solely on initial HCG level; follow-up is essential to assess for appropriate rise or fall 4
• Always use the same laboratory for serial HCG measurements to ensure consistency, as different assays have varying sensitivities 5
• The traditional discriminatory threshold of 3,000 mIU/mL has virtually no diagnostic utility for predicting ectopic pregnancy, with a positive likelihood ratio of 0.8 and a negative likelihood ratio of 1.1 4