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Serum Pregnancy Tests

Test code(s) 8396(X), 19485(X), 19720, 90378

Question 1. What is measured?

Serum pregnancy tests measure human chorionic gonadotropin (hCG) which is produced by placental syncytiotrophoblasts following implantation. hCG is composed of 2 subunits: alpha and beta. The hCG, Total, Quantitative test (test code 8396[X]) detects intact hCG molecules and free beta-subunits of the hCG molecule.

Question 2. How soon after conception can an hCG test be positive?

The serum hCG test can be positive as early as 7-10 days following conception.

However, in some pregnant women, the test is not positive 7-10 days after conception. This is because the time to implantation (and production of hCG) can be longer in some women. So a negative result does not rule out pregnancy, and repeat testing may be indicated.

Question 3. Can a nonpregnant woman have a positive serum hCG result?

Yes, there are several situations in which a positive serum hCG test may be seen in a nonpregnant woman:

  • The pituitary secretes hCG, especially around the time of menopause.
  • The woman has a biochemical pregnancy: the hCG test is positive in the absence of other signs of pregnancy. This occurs when there is a very early first trimester pregnancy loss, and hCG has not yet declined to an undetectable level.
  • The woman has an hCG-producing tumor such as a gestational trophoblastic tumor or germ cell tumor.
  • The woman has received exogenous hCG.
  • Heterophile antibodies caused the false-positive result, sometimes called a phantom hCG. When suspected, further evaluation may include 1) a urine pregnancy test, which is often negative in these cases; 2) a serum hCG test performed using a different testing platform (hCG, Alternate Method, test code 90378); and 3) an hCG test designed to detect possible heterophilic antibody interference (hCG, Total, with HAMA Treatment, test code 19720).

Question 4. Can a pregnant woman have a negative serum hCG test result?

Yes, if the test is performed before implantation occurs (ie, test performed too early). See Question 2.

Question 5. Are there other causes of a false-negative serum hCG test result?

Yes. Very high concentrations of hCG, as seen in gestational trophoblastic disease, can cause a false-negative result or a lower-than-expected result. This is due to an analytical phenomenon called the hook effect. In these cases, specimen dilution should result in an accurate hCG measurement.

Question 6. What are some other uses of the serum hCG test?

Other uses include 1) diagnosis of pregnancy abnormalities such as ectopic pregnancy, molar pregnancy, and other types of gestational trophoblastic disease; 2) diagnosis and management of germ cell tumors; and 3) prenatal screening for chromosomal abnormalities such as Down syndrome.

 

References
 
  1. Jacobs DS, DeMott WR, Oxley DK. Jacob's & DeMott Laboratory Test Handbook. 5th ed. Hudson (Cleveland), OH: Lexi-Comp, Inc.; 2001.
  2. McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011.
This FAQ is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
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