Chorionic gonadotropin is a hormone consisting of two polypeptide chains or subunits designated alpha and beta. The alpha chain is identical to the alpha chain of luteinizing hormone (LH), follicle stimulating hormone (FSH), and thyroid stimulating hormone (TSH). The beta chain is identical to that of LH except for the C-terminal end, which contains an additional 24 amino acids. Antibodies made against the alpha subunit will cross-react with LH, FSH, and TSH, but antibodies can be made to the beta subunit that are hCG-specific. All tests for pregnancy utilize antibodies to both subunits, which makes the pregnancy test highly specific for hCG. Chorionic gonadotropin is produced at an exponential rate through week 12 of gestation, often reaching in excess of 100,000 mIU/mL. In a normal pregnancy, the production of hCG doubles approximately every two days during this period. The level falls off sharply after the first trimester to approximately 20,000 mIU/mL, and is maintained at this level throughout a normal pregnancy. Following a normal delivery, the hCG can be detected in serum and urine for three to four weeks. This period may be longer following an aborted pregnancy, especially if a trophoblastic tumor was present.
All pregnancy tests are double antibody sandwich immunoassays. The most commonly used platform, called immunochromatography, consists of a disposable device containing a membrane on which an antibody to one hCG subunit is immobilized. The membrane also contains an antibody to the other hCG subunit that is mobile. The mobile antibody is conjugated to an enzyme, dyed latex particle, or colloidal gold particle. Sample is added to the device and is drawn by capillary action onto the membrane, where it mixes with the mobile antibody. If hCG molecules are present, they bind to the mobile antibodies, forming antibody-antigen complexes. These migrate along the membrane to the region containing the immobilized antibody. The immobilized antibody binds to the other hCG subunit, forming an antibody-hCG-antibody sandwich that remains fixed to the membrane in the reaction zone region. At this point, the dye or gold particles are focused in the reaction zone and produce color, usually in the form of a plus sign or other visible indicator of a positive test. If an enzyme-conjugated antibody is used, a substrate solution is added, which is hydrolyzed by the enzyme to produce a colored product at the reaction zone.
Victoria DeMoranville, The Gale Group Inc., Gale, Detroit,