From the earliest stage of development, the placenta produces hormones, either on its own or in conjunction with the fetus. The very young placental trophoblast produces appreciable amounts of the hormone human chorionic gonadotropin (hCG), which is excreted in the urine. This hormone is not found in the urine of men or of healthy, young, nonpregnant women.
Increased urinary hCG levels form the basis of the tests for pregnancy; hCG is present in blood and urine whenever there is living chorionic/placental tissue. hCG is made up of α- and β-subunits. The β-subunit is the most sensitive and specific test for early pregnancy. hCG can be detected in the urine of pregnant women 26–36 days after the first day of the last menstrual period (i.e., 5–7 days after conception). Pregnancy tests should return to negative 3–4 days after delivery.
Positive: Pregnancy exists.
Negative: Pregnancy does not exist.
Over-the-counter (home) pregnancy kits can typically detect hCG in the urine soon after the first missed menstrual period.
Collect an early morning urine specimen. The first morning specimen generally contains the greatest concentration of hCG. A random specimen may be used, but the SG must be >1.005.
Do not use grossly bloody specimens. If necessary, a catheterized specimen should be used.
A positive result usually indicates pregnancy.
Positive results also occur in:
Choriocarcinoma
Hydatidiform mole
Testicular and trophoblastic tumors in males
Chorioepithelioma
Chorioadenoma destruens
About 65% of ectopic pregnancies
Negative or decreased results occur in:
Fetal demise
Abortion, threatened miscarriage (test remains positive for 1 week after procedure)
False-negative test results and falsely low levels of hCG may be caused by dilute urine (low SG) or by using a specimen obtained too early in pregnancy.
False-positive tests are associated with
Proteinuria
Hematuria
The presence of excess pituitary gonadotropin
Certain drugs (e.g., chlorpromazine, phenothiazines, methadone)