HCG REFERENCE SERVICE
POTENTIAL SOURCES OF HCG IN AND OUTSIDE OF PREGNANCY

 

The hCG Reference Service investigates the nature of the hCG present in parallel serum and urine samples. Multiple forms of hCG (intact hCG, nicked hCG, hyperglycosylated or carbohydrate-variant hCG, asialo hCG, free ß-subunit or ß-core fragment) may arise from the synthesis and degradation of hCG subunits (click here to see illustration of the synthesis and degradation of hCG). From the nature of the hCG in serum and urine, published information, and from our data bank, inferences are made about the origin of the hCG-related molecules (pregnancy, pituitary hCG, hydatidiform mole, persistent trophoblast disease, choriocarcinoma, germ cell or other cancer, phantom or false-positive hCG) (see references, below).
    Potential sources of hCG and related molecules are listed (see references, below) -

1) Normal pregnancy
    Predominantly intact hCG is present in serum and urine samples. The concentration of intact hCG may reach up to 150,000 mIU/ml in both serum and urine. hCG levels rise exponentially for the first 8 weeks of pregnancy, reaching a peak at 10 weeks after the last menstrual period. In the following 10 weeks levels slowly decline to approximately one fifth of peak levels and remain around one fifth of peak levels until term (click here to see data on hCG levels during normal pregnancy). Individual hCG level can vary widely, with as much a 20-fold variation in individual levels (click here to see data on hCG levels during normal pregnancy).
    In addition to intact hCG, a low and variable proportion of free ß-subunit (<1% of intact hCG level) nicked hCG (<10% of intact hCG level) and hyperglycosylated hCG are present in serum and urine throughout pregnancy. ß-core fragment is present only in urine samples at concentrations ranging from one fifth (early pregnancy) to 5 times (at term) the concentration of hCG.

2) Early pregnancy loss (EPL) or biochemical pregnancy    
    hCG may also be produced by an EPL or "biochemical pregnancy". This is an embryo which fails to implant properly in the uterus, or is rejected by the uterus. It is followed by a normal or slightly heavier than normal menstrual period. This may be a molar pregnancy (see hydatidiform mole, below) or grossly abnormal embryo. hCG levels may rise in the week following implantation (second week of conception) like a normal term pregnancy (click here to see data on hCG levels during normal pregnancy). hCG concentration reaches a peak after 2 weeks conception (28 days after start of last menstrual period) at 10 to 100 mIU/ml, then rapidly declines. Normal or slightly heavier than normal menses follow. Most women are unaware that they have had an EPL. EPLs are quite common (reports vary, indicating similar to lesser incidence than normal pregnancies). EPLs are a cause of false-hopes of pregnancy. Individuals may get a positive result using a home pregnancy or laboratory pregnancy test in the last week of their menstrual cycle (26-29 days after start of last menstrual period). When the test is repeated a few days later a negative result or much lower concentration of hCG is detected. It is because of EPLs that we recommend that pregnancy tests be performed no earlier that day 31 since the start of last menstrual cycle, or if done earlier, are repeated on or after day 31 since the start of last menstrual cycle. This assures detection of a potentially viable or clinical pregnancy.  

3) Ectopic or extrauterine pregnancy
    In women with extrauterine or ectopic pregnancies, unduly low hCG levels may be detected. For any particular gestation age (6, 7, 8, or 9 weeks since last menstrual period) the median hCG levels in serum or urine may be one fifth (6 weeks) to one fiftieth (8 weeks) of the median levels in normal intrauterine pregnancies. In these pregnancies, almost entirely intact hCG is present in serum and urine samples, with much lesser proportions of hCG degradation products, nicked hCG, free ß-subunit and ß-core fragment (in urine). Very low hCG levels are suggestive of ectopic pregnancy. It must be emphasized, however, that they are only suggestive. Significant overlap exist between the ranges of hCG levels in ectopic pregnancies and normal pregnancies (click here to see data on hCG levels during normal pregnancy). A more accurate diagnostic tool for identifying ectopic pregnancy is hCG doubling rate. In the first 8 weeks of pregnancy hCG levels should double every 2 days. The combination of low hCG levels, and inability of levels to increase or double over a 2 day period is a stronger diagnostic indicator of ectopic pregnancy.    

4) Gestational Down syndrome
    Slightly higher than normal serum hCG and serum free ß-subunit levels are associated with pregnancies with Down syndrome fetus. The median serum hCG and free ß-subunit level in a Down syndrome pregnancy is twice the level in a normal pregnancy. Considering the wide variance in individual hCG levels (click here to see data on hCG levels during normal pregnancy), and the rarity of pregnancies with Down syndrome fetuses, higher than normal hCG or free ß-subunit levels are not a clear indicator of genetically abnormal pregnancy. Instead, a combination of magnitude of hCG or free ß-subunit, gestational age, and several other pregnancy-associated biochemical tests (alpha-fetoprotein, unconjugated estriol, and inhibin) are used together to predict risk for having a Down syndrome pregnancy in the second trimester. If you are found to be at high risk for Down syndrome by these tests, amniocentesis is suggested. A new test, hyperglycosylated hCG (invasive trophoblast antigen or ITA), may be a more accurate indicator of genetically-abnormality in the second trimester of pregnancy. Hyperglycosylated hCG levels (<3% of the total hCG-related molecule concentration) are not 2-fold, but 9-fold elevated in Down syndrome pregnancies.

5) Hydatidiform mole
    Predominantly intact hCG is present in serum and urine samples. Prior to evacuation of a hydatidiform mole, intact hCG concentration may be as high as 2,000,000 mIU/ml. A slightly higher  proportion of free ß-subunit (1-10% of intact hCG level) may be present in serum and urine in patients with hydatidiform moles. In the months after evacuation of a hydatidiform mole, hCG can become totally nicked, or replaced with nicked hCG (when hCG levels drop below 100 mIU/ml). A small, but significant proportion of hyperglycosylated hCG molecules may be detected in serum and urine (up to 10% of intact hCG level), particular in cases of persistent trophoblastic disease (up to 20% of intact hCG level).

6) Persistent trophoblast disease or persistent mole
    Variable levels of hCG, mostly intact hCG, may be present in serum and urine samples from individuals with persistent trophoblastic disease (dependent upon the amount of trophoblast mass). Persistent trophoblastic disease may be associated with higher than normal proportions of free ß-subunit (2-10% of intact hCG level), higher proportion of nicked hCG after therapy or surgery (up to 100% of hCG molecules), and possibly with higher proportions of hyperglycosylated hCG (up to approximately 40% of hCG molecules).  

7) Choriocarcinoma
    Nicked hCG and hyperglycosylated hCG may predominate in the serum and urine of  individuals with post-molar or post-gestational choriocarcinoma. Unduly high proportions of free ß-subunit may also be detected in serum and urine (5-40% of intact hCG) in some choriocarcinoma cases.

8) Germ cell, bladder and other non-trophoblastic malignancies    
    Non-trophoblastic cancer cells, germ cell tumors (dysgerminoma and testicular tumors), bladder cancer, ovarian cancer and certain other malignancies may generate a small amount of hCG alpha and beta subunit. Commonly, the amount of subunit is insufficient for combination to occur to make intact hCG. In these cases, variable concentrations, commonly very low concentrations (<100 mIU/ml) of free ß-subunit is present in the serum, and low concentration of predominantly ß-core fragment (the terminal breakdown product of ß-subunit) in urine samples. The free ß-subunit in serum may be detected by total hCG assays (detect intact hCG and free ß-subunit). Often the most sensitive marker for these disease may be ß-core fragment in urine.

9) No pregnancy, no cancer, no evidence of disease, pituitary hCG production    
    The gonadotroph cells of the pituitary (those that make lutenizing hormone) normally produce a miniscule amount of hCG and hCG ß-core fragment (<0.5 mIU/ml). Occasionally, in individuals post-menopause, in cases with pituitary tumor, or rarely in normal menstruating women, elevated levels of hCG (up to 20 mIU/ml) and ß-core fragment can derive from the pituitary. Pituitary hCG differs from normal trophoblast hCG in having sulfated oligosaccharide side chains, with limited sialic acid residues.

10) No pregnancy, no cancer, no evidence of disease, phantom or false positive hCG production
    Antibodies generated in the body against other human antibodies (IgG against IgG) may bind both human and animal antibodies (heterophilic antibodies). These may interfere with commercial hCG tests, by causing phantom hCG or false-positive hCG results (10 to 400 mIU/ml) (click here to see the principal of false positive or phantom hCG tests). Surgery and chemotherapy are sometimes performed for ectopic pregnancy or post-gestational choriocarcinoma solely on the basis of phantom or false positive hCG test data.
    The interfering antibodies are present in serum but not urine samples. They may variably affect different commercial hCG assays depending upon the species and type of the antibodies used and the antibody concentration.  Phantom hCG may be detected by demonstrating the presence of significant hCG (or related molecule) immunoreactivity, with complete absence in urine samples. Phantom hCG can be confirmed by the demonstration of loss of the hCG activity after treatment with heterophilic antibody blocking agents, by the finding of widely variable results in different commercial hCG tests, and by the finding of other false positive tests with serum samples  (click here to see the principal of false positive or phantom hCG tests).

REFERENCES

1.    Cole, L.A., Kardana, A., Park S-Y., Braunstein, G. The deactivation of hCG by nicking and dissociation. J Clin Endocrinol Metab 76:704-10, 1993.

2.    Kardana, A., Cole, L.A. Human chorionic gonadotropin b -subunit nicking enzymes in pregnancy and cancer patient serum. J. Clin. Endocrinol. Metab.,79:761-767, 1994

3.    Cole, L.A., Tanaka, A., Kim, G.S., Park, S-Y., Koh, M.W., Schwartz, P.E., Chambers, J.T., and Nam, J-H. Beta core fragment (b -core / UGF / UGP), a tumor marker: Seven year report. Gynecol. Oncol., 60:264-270, 1996.

4,    Cole L. Immunoassay of hCG, its Free Subunits and Metabolites. Clin Chem 43:2233-2243, 1997.

5.    Cole LA. Phantom hCG and phantom choriocarcinoma. Gynecol Oncol, 71:325-329, 1998.

6.    Cole, L.A. Shahabi, S., Rinne, K.M., Oz, U.A., Bahado-Singh, R.O., Mahoney, M.J. Urinary Screening Tests for Fetal Down Syndrome: II. Hyperglycosylated hCG. Prenat Diagn, 19:340-349, 1999.

7.     Elliott M, Kardana A, Lustbader J, Cole L. Carbohydrate and peptide structure of the alpha- and beta-subunits of hCG from normal and aberrant pregnancy and choriocarcinoma. Endocrine 7:15-32, 1997.

8.    Alfhan, H., Haglund, C., Roberts, P., and Stenman, U.H. Elevation of free ß-subunit of human choriogonadotropin and core b fragment of choriogonadotropin in serum and urine of patients with malignant pancreatic and biliary disease Cancer Res. 52, 4628-4633 (1992).

9.    Cole L, Kohorn E, Kim G. Detecting and monitoring trophoblast disease: New perspectives in measuring hCG levels. J Reprod. Med. 39:193-200, 1994.

10.   Cole, L.A., Rinne, K.M., Shahabi, S., and Omrani, A. False positive hCG levels leading to unnecessary surgery and chemotherapy, and needless occurrences of diabetes and coma. Clin Chem, 45:313-314, 1999.

11.   Birken, S., Maydelman, Y., Gawinowicz, M.A., Pound A., Liu Y., Hartree A.S., Isolation and characterization of human pituitary chorionic gonadotropin. [Journal Article] Endocrinology. 137:1402-1411, 1996.

12.  Cole, L.A., Kardana, A., Seifer, D.B., and Bohler, H.C. Urine hCG b-subunit core fragment, a sensitive test for ectopic pregnancy. J. Clin. Endocrinol. Metab., 78:497-499, 1994. Cole, L.A., Kardana, A., Seifer, D.B., and Bohler, H.C. Urine hCG b-subunit core fragment, a sensitive test for ectopic pregnancy. J. Clin. Endocrinol. Metab., 78:497-499, 1994.

13.  Cole, L.A. hCG, nicked hCG, free subunits and fragments in pregnancy, trophoblast disease and cancer. Clin. Chem. 41:26-27, 1995.

14.  Cole, L.A., Down’s syndrome screening using urine ß-core fragment test; Choice of immunoassay. Prenatal Diagnosis, 25:679-682, 1995.

15.  Cole, L.A., Omrani, A., Cermik, D., Bahado-Singh, R.O., and Mahoney, R.O. Hyperglycosylated hCG, a potential alternative to hCG in down syndrome screening. Prenat. Diagn., 18:926-933, 1998.

16.  Knight, G.J., Palomaki, G.E., Neveux, L.M., Fodor, K.K., Haddow, J.E. (1998). hCG and the free ß-subunit as screening test for Down syndrome. Prenat Diagn 18:235-245.

19.  Canfield RE, O'Connor JF, Birken S, Krichevsky A, Wilcox AJ. Development of an assay for a biomarker of pregnancy and early fetal loss. Environ Health Persp 1987; 74:57-66.