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The measurement of human chorionic gonadotropin for pregnancy testing.


 Human chorionic gonadotropin (HCG) has had the widest application among serveral possible endocrine markers for detecting and monitoring early pregnancy because it normally originates only in trophoblastic tissue, it rises exponentially during early pregnancy as a reflection of trophoblast function, and it is detectable with increased sensitivity and specificity by available analytical methods. HCG is composed of 2 non-covalently bound subunits designated alpha and beta; the beta subunit is biologically and immunologically unique to HCG. HCG exhibits extensive homology in the beta subunit to luteinizing hormone (LH), and both LH and HCG bind with equal affinity to identical receptors in vivo. HCG is detectable by radioimmunoassay (RIA) in the nonpregnant state and in the preimplantation phase of pregnancy. Beginning shortly after implantation, approximately 8 days after presumed ovulation, HCG begins to rise rapidly, reaching a peak 8-12 weeks after the last menstrual period, after which it plateaus. Measurement of HCG levels for pregnancy testing has several limitations, including substantial daily variation in HCG levels in individual women, difficulty in interpreting whether a single value is normal for gestational age, and the length of the biological half-life of intact HCG. The urine concentration of HCG parallels but is not identical to the serum concentration. The UCG-slide test is sensitive to 2000 mlU/ml urine and the UCG-tube test is sensitive to 500-1500 mlU/ml of urine. Both are positive 5-12 days after the missed period. However, LH in menopausal women will occasionally give a positive result and certain drugs may interfere. Radioreceptor assays are sensitive to HCG in serum 14 days after conception. The test will detect most pregnancies by the time of expected menses. Serum protein, drugs, and normal LH surges do not significantly interfere. A qualitative radioimmunoassay using antisera directed toward the unique carboxy-terminal end of the beta subunit of HCG may detect pregnancy 7-10 days after conception. The cost and turnaround times of pregnancy tests vary with their sensitivity. Usually a tube test and confirming examination are sufficient. Quantification of HCG may be useful in diagnosing ectopic pregnancy, providing a prognosis in threatened abortion, and following neoplasms.
