Analytical Techniques

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.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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