HCG is composed of a and b subunits. The HCG a subunit is indistinct from the a subunit of several other glycoproteins, including luteinizing hormone. The b subunit is unique to HCG. Qualitative enzyme-linked immunosorbent assays (ELISAs) that detect b-HCG in urine can be completed in minutes at the patient's bedside. Available commercial tests can detect b-HCG concentrations as low as 10 to 20 mIU/mL. These tests have a false-negative rate of only 1 percent in detecting pregnancy as early as 1 week after conception. However, to achieve this level of sensitivity, the analysis must be performed on a urine specimen that is not dilute. When urine is dilute, a false-negative result may be obtained for women with early pregnancy (serum HCG < 50 IU/mL). 9 Serum analysis should be performed in clinical situations when pregnancy is a concern but the urine result is questionable. Quantitative serum values are obtained by ELISA technique and can be completed in 1 to 2 h. There are several international standards for serum b-HCG, and the specific reference range should be appropriate for the standard employed.
A positive result on a pregnancy test, whether qualitative or quantitative, does not confirm a normal intrauterine pregnancy. Ectopic pregnancy, recent spontaneous or induced abortion, and HCG-secreting tumors may also produce a positive result. Single quantitative b-HCG determinations in combination with pelvic ultrasonography may differentiate intrauterine pregnancy from these conditions.10 Serial quantitative serum b-HCG determination may be useful in some outpatient situations, since serum b-HCG levels double every 1.4 to 2.0 days following implantation in early normal pregnancy. Failure of the hCg concentration to double in this time period suggests an ectopic pregnancy or a nonviable pregnancy.
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