The diagnosis of pregnancy is central to the diagnostic workup of possible EP. When a patient is not pregnant, there is no ectopic pregnancy. Pregnancy tests currently in use rely on the detection of the b subunit of human chorionic gonadotropin (bhCG) in the urine or serum. Human chorionic gonadotropin (hCG) is a hormone produced by the trophoblast and is normally undetectable in the serum. Intact hCG consists of the a and b subunits. Tests based on detection of the intact molecule or the a subunit cross-react on immunologic assays with hormones found in the nonpregnant individual and are thus less specific then tests for the bhCG subunit.
Human chorionic gonadotropin preparations are currently standardized in relation to the International Reference Preparation (IRP). Other standard preparations are not equivalent. A preparation often referred to in earlier literature is the Second International Standard (2nd IS). The IRP is roughly equal to 1.7 times the 2nd IS. To avoid confusion when interpreting the literature, attention must be paid to the standard used. In this chapter, hCG and bhCG concentrations are in reference to the IRP unless otherwise noted.3
Very early in either an intrauterine pregnancy (IUP) or an EP, detectable amounts of bhCG are released into the serum and filtered into the urine. The concentration of bhCG is fairly closely correlated in the urine and serum, with urinary concentration also depending upon urine specific gravity. Qualitative urine and serum tests for pregnancy usually use the enzyme-linked immunosorbent assay (ELISA) methodology. In the laboratory setting, ELISA tests can detect bhCG at concentrations below 1 mIU/mL. Qualitative tests in clinical use are typically reported as "positive" when the bhCG concentration is 320 mIU/mL in urine and 310 mIU/mL in serum. A "positive" qualitative test therefore implies that bhCG is present in at least this concentration. At this level of detection, the false-negative rate for detection of pregnancy will not be more than 1 percent for urine and 0.5 percent for serum and may be less. In clinical use, the performance of urine qualitative testing has been found to be from 95 to 100 percent sensitive and specific as compared with serum tests. One study showed that in 100 percent of 95 patients with EP, pregnancy was correctly diagnosed with a sensitive qualitative urine test (Hybritech Tandem ICON II). 4 Urine tests can be performed rapidly at the bedside, and kits from some manufacturers may be used for either urine and serum. A bedside test for urine is therefore the best first test for pregnancy when EP is suspected. Dilute urine may cause a false-negative urine pregnancy test.5 This is most likely to occur early in pregnancy when bhCG levels are low (<50 mIU/mL). If urine is not available, use of a bedside serum test may be considered. When a bedside urine test is negative and EP is still being considered, a quantitative serum test should be performed. The sensitivity of quantitative serum testing for the diagnosis of pregnancy is virtually 100 percent when an assay capable of detecting 35 mIU/mL of bhCG is used. 45
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