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Human chorionic gonadotropin (hCG) is a glycoprotein secreted by the placenta during pregnancy that consists of an alpha and beta subunit. The alpha subunit is structurally similar to the alpha subunits of FSH, LH and TSH. The beta subunit is distinct for hCG. The release of hCG into maternal circulation begins with embryo implantation 5 to 7 days after fertilization. Several forms of hCG are present in serum and urine (Clinical Chemistry 1997; 43:2233 – 43). In early pregnancy, trophoblast cells of the placenta secrete predominantly intact hCG. This molecule is broken down into several degradation products. The normal degradation pathway is: Non-nicked hCG-à free alpha subunit & nicked free beta subunità beta core fragment. The proportion of nicked hCG can vary between 0 & 59% during the first trimester. In late pregnancy, the production of free alpha subunit increases.
The serum of patients with uncomplicated pregnancies contains
Commercial assays use any of 7 antibody combinations that may detect:
Quantitative serum hCG tests have a sensitivity of less than 10IU/L and can detect pregnancy two to four days earlier than a urine test. Healthy, non-pregnant women have serum hCG levels < 5 IU/L. Serum hCG values > 25 IU/L indicate pregnancy. Levels between 5 and 25 IU/L often indicate early pregnancy, but results need to be interpreted cautiously because false positive results can occur in this range. In this situation, the test should be repeated 48 hours later to confirm pregnancy. Serum hCG concentrations double every 1.5 to 2 days during the first 6 weeks of gestation in patients with uncomplicated intrauterine pregnancies. Serum hCG levels can be used to estimate gestational age. Values for hCG generally peak between 8 to 12 weeks gestation and then gradually decline throughout the remainder of pregnancy.
Estimates may differ by as much as 2 weeks from predictions based on menstrual history. Gestational dating by ultrasound is more accurate. Serum hCG levels increase in perimenopausal (41-55 years) and postmenopausal (>55 years) women, because decreases in ovarian estrogen and progesterone production result in a lessening of the negative feedback control of gonadotropin releasing hormone (GnRH) by the hypothalamus. As a result, continuous GnRH stimulation of gonadotrope cells in the pituitary leads to increased LH and FSH production as well as pituitary hCG. Consequently, serum hCG levels as high as 8 IU/L and 15 IU/L may occur in nonpregnant perimenopausal and postmenopausal women, respectively. The hCG level should exceed 20 IU/L for pregnancy to be considered in this age group. Quantitative serum hCG levels are helpful in evaluating suspected ectopic pregnancy because the classical triad of abdominal pain, delayed menses, and vaginal bleeding occurs in less than 20% of cases. Most ectopic pregnancies produce sufficient amounts of hCG to test positive with urine pregnancy tests. A urine pregnancy test can confirm that a woman is pregnant much faster and cheaper than a serum quantitative hCG. Serum hCG quantitation can then be done to determine if a fetus is large enough to be visualized by ultrasound. Gestational sacs are visible by transvaginal ultrasonography when serum hCG levels equal or exceed 1600 mIU/mL. If the hCG level is this high and no sac is visible in the uterus, ectopic pregnancy is suspected. Quantitation of serum hCG levels in paired specimens drawn 48 hours apart can also be used to help diagnose ectopic pregnancy. Women with uncomplicated pregnancies increase their serum hCG levels at least 1.6 fold during this interval. Smaller increments are consistent with ectopic or complicated pregnancy. The circulating half-life of hCG is 24 hours. hCG may still be detectable in maternal serum 8 to 24 days after an uncomplicated vaginal delivery. Following a first trimester spontaneous abortion, serum hCG may be detectable up to 60 days. A single serum hCG level 16 days after ovulation in women who became pregnant through assisted reproductive technology provides a useful predictor of pregnancy outcome. hCG levels above 500 IU/L predict a greater than 95% chance of ongoing pregnancy. Levels between 25 and 50 IU/L are associated with a less than 35% probability of ongoing pregnancy. (Fertil Steril 2000;73:260-74). Urine pregnancy tests are reported as positive or negative. Reference value is negative. Serum hCG reference range is:
Specimen requirement for the urine pregnancy test is 10 mL of a first morning midstream urine collection in a urine specimen cup. Specimen requirement for quantitative serum hCG is one SST tube of blood. |
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