Human Chorionic Gonadotropin (hCG): Reference Range, Interpretation, Collection and Panels

Description

Human chorionic gonadotropin (hCG) is a dimer consisting of a 145 amino acid beta-subunit that is unique to hCG and a 92 amino acid alpha-subunit. The alpha-subunit is identical to that for luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH). The alpha and beta-subunits have separate genes on separate chromosomes (chromosomes 6 and 19, respectively). After synthesis, the alpha and beta-subunits are bonded with a noncovalent bond before being released into the circulation.

Carbohydrate side chains comprise 25-40% of the molecular weight of hCG. The alpha-subunit contains 2 N-linked oligosaccharides; the beta-subunit contains 2 N-linked oligosaccharides, as well as 4 O-linked oligosaccharides on the C-terminal extension. The 2 most common forms of hCG synthesized by cells are regular hCG and hyperglycosylated hCG (hCG-H). HCG-H contains more sugar residues than regular hCG: 1.5-fold more sugar residues on the N-linked oligosaccharides (16 versus 11 sugar residues for regular hCG) and 2-fold more sugar residues on O-linked oligosaccharides.

Indications/Applications

Gestational and nongestational trophoblasts are by far the most common sources of hCG, but a small amount of the hormone may also be produced by the pituitary gland and nontrophoblastic malignancies.

Pregnancy

The syncytiotrophoblast covers the villous tree and has several functions, such as transport of gases, nutrients, and waste products and synthesis of peptide and steroid hormones that regulate placental, fetal, and maternal systems. The syncytiotrophoblast produces regular hCG, which promotes progesterone production by the corpus luteum until placental progesterone production becomes established (after 6 weeks of gestation). Regular hCG also appears to play a role in myometrial spiral artery angiogenesis.

The level of free beta-subunit of hCG is often determined in pregnant women as part of maternal serum screening for Down syndrome. The free beta-subunit variants measured depend on the assay. Increased total hCG levels in the first and second trimester are associated with Down syndrome, while decreased levels may occur in trisomy 18. Elevations of hCG can also occur in multiple pregnancies, singleton pregnancies in which the gestational age has been overestimated, triploidy, fetal loss, and hydrops fetalis. ^{[4, 5]}

Serum total hCG concentration peaks at about 93,000 mIU/mL (range 27,300-233,000 mIU/mL) at 8- 11 weeks of gestation.

Gestational trophoblastic disease

HCG promotes trophoblast growth and invasion. The invasion may be controlled, as in implantation of pregnancy and complete/partial mole, or malignant, as in the invasive gestational trophoblastic diseases, choriocarcinoma and invasive mole.

Complete and partial mole

Regular hCG is the principal form of hCG associated with complete and partial hydatidiform mole.

A total hCG result greater than 100,000 mIU/mL strongly suggests complete hydatidiform mole, although the peak hCG in many normal pregnancies can reach this level.

Partial moles produce less regular hCG than complete moles. In the USA hCG Reference Service experience, the median hCG concentration in 21 partial moles was about 49,000 mIU/mL (range 11,600-220,114 mIU/mL), and only 14% of cases exceeded 100,000 mIU/mL.

Invasive mole and choriocarcinoma

In gestational trophoblastic neoplasias, such as invasive mole and choriocarcinoma, a high level of hCG can help to distinguish invasive from noninvasive mole, although some overlap occurs in their normal ranges.

Serial measurement of hCG levels is standard follow-up of women diagnosed with complete or partial mole. An increasing or plateauing level of total hCG is diagnostic of invasive disease (invasive mole or choriocarcinoma).

Choriocarcinoma

Choriocarcinoma consists of sheets of anaplastic cytotrophoblasts and syncytiotrophoblasts without chorionic villi. Some intermediate trophoblasts may also be seen. HCG levels approaching 600,000 mIU/mL are seen. The level of hCG correlates with tumor mass.

Pituitary gland

The normal pituitary gland produces a small amount of hCG. The level of hCG attributable to pituitary production ranges from 1-32 mIU/mL.

Pituitary production of hCG is most notable around the time of menopause (natural or surgical) and prior to ovulation, which are times when LH levels peak. One possible explanation is that a small amount of hCG is produced along LH because the single LH beta-subunit gene is buried among the 7 back-to-back hCG beta-subunit genes.

Return of the serum hCG concentration to undetectable following pregnancy termination varies widely from 7-60 days. The period of time depends primarily upon the hCG concentration at the time of termination. The hCG concentration peaks at 8-11 weeks at approximately 90,000 mIU. This is in contrast with term pregnancy, for which the hCG concentration is lower. The decline in serum hCG is rapid for the first several days (half-life 9-31 hours) and then proceeds more slowly (half-life 55-64 hours).

Nongestational malignancies

Outside of pregnancy, hCG may be secreted by abnormal germ cell, placental, or embryonal tissues, in particular seminomatous and nonseminomatous testicular tumors; ovarian germ cell tumors; gestational trophoblastic disease (GTD, hydatidiform mole, and choriocarcinoma); and benign or malignant nontesticular teratomas. Rarely, other tumors including hepatic, neuroendocrine, breast, ovarian, pancreatic, cervical, and gastric cancers may secrete hCG, usually in relatively modest quantities

During pathologic hCG production, the highly coordinated secretion of alpha and beta subunits of hCG may be disturbed. In addition to secreting intact hCG, tumors may produce disproportionate quantities of free alpha subunits or, more commonly, free beta subunits. Assays that detect both intact hCG and free beta hCG, including this assay, therefore, tend to be more sensitive in detecting hCG-producing tumors.

With successful treatment of hCG-producing tumors, hCG levels should fall, with a half-life of 24-36 hours, and eventually return to within normal limits.

Considerations

An elevated HCG level may be physiologic, pathophysiologic from a tumor or artifactual from a false-positive hCG test. Unless a tumor is evident, excluding these possibilities before initiating chemotherapy for assumed persistence of disease is essential.

A false-positive HCG test may be caused by naturally occurring cross-reacting antibodies that interfere with the test. The capture and tracer antibodies used for hCG testing may be goat, sheep, or rabbit polyclonal antibodies or mouse, goat, or sheep monoclonal antibodies. Humans extensively exposed to animals or certain animal byproducts can develop human antibodies against these animal antibodies (HAAA). In addition, humans naturally generate human anti-human immunoglobulin antibodies that can cross-react with and bind animal antibodies; these are called heterophilic antibodies.

Also, recent infections or exposure to mononucleosis can produce these HAAAs; those with IgA deficiency syndrome also often have heterophilic antibodies. Each human antibody is bivalent, so if an HAAA or heterophilic antibody is present in a person's serum, it can bind and link together the immobilized and tracer antibodies of the hCG test, making an immobilized capture antibody-heterophilic antibody/HAAA-tracer antibody sandwich, which results in a false positive hCG result. The false positive HCG test can lead to the misdiagnosis of cancer and sometimes lead to needless surgery and chemotherapy.

To prevent false testing, animal serum and nonspecific animal antibodies are added to HCG assays with capture antibody, tracer antibody, and other components. This excess of nonspecific antibodies overwhelming saturates heterophilic antibodies and HAAA in human serum samples and usually eliminates their interference with the assay. This method does not always work, however, and false positive cases still occur. This understandably creates confusion, as some false positive cases are incorrectly interpreted as recurrence of gestational trophoblastic disease.

Two main methods for identifying false-positive hCG tests exist are as follows:

• The most readily available approach is to show the absence of hCG in the patient's urine. A true hCG elevation should be present in both serum and urine. Urine HCG is usually never detected in patients with false-positive serum hCG tests. A sensitive urine hCG test should be used to avoid missing low-level true-positive tests. Alternatively, to detect low concentrations of hCG, urine samples can be tested on the same instrument used to quantitate hCG in serum samples; these tests are typically sensitive. Avoid using a point-of-care urine test, as these tests tend to be insensitive to low levels of hCG.

• A second useful way of identifying a false-positive serum hCG result is to send the serum to 2 laboratories using different commercial assays. If the assay results vary greatly or are negative in one or both alternative tests, then a false-positive hCG can be presumed.

Patients who have false-positive hCG test results are also at risk for other false positives, such as CA-125 and thyroid antibodies. They should make their future health care providers aware of this problem and it should be noted in their medical records.