Urine Pregnancy Test- Human Chorionic Gonadotropin (HCG) Testing

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 normal, young, non-pregnant 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 a glycoprotein made up of alpha and beta subunits with 237 amino acids. The beta subunit is the most sensitive and specific test for early pregnancy. hCG can be detected in the urine of pregnant women 26 to 36 days after the first day of the last menstrual period (ie, 5 to 7 days after conception). Pregnancy tests should return to negative 3 to 4 days after delivery. 

Reference Values 

Normal 

Positive: pregnancy exists 

Negative: non-pregnant state 

Procedure 

1. 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 more than 1.005. 
2. Do not use grossly bloody specimens. If necessary, a catheterized specimen should be used. 

Clinical Implications 

1. A positive result usually indicates pregnancy.
2. Positive results also occur in:

• Choriocarcinoma 
• Hydatidiform mole 
• Testicular and trophoblastic tumors in males 
• Chorioepithelioma 
• Chorioadenoma destruens 
• About 65% of ectopic pregnancies 

    3. Negative or decreased results occur in:

• Fetal demise 
• Abortion, threatened abortion (test remains positive for 1 week after procedure) 

Interfering Factors 

1. 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. 
2. False-positive tests are associated with:

• Proteinuria 
• Hematuria 
• The presence of excess pituitary gonadotropin 
• Certain drugs (eg, chlorpromazine, phenothiazines, methadone)