Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Most often, this test is performed to confirm or rule out pregnancy. Serum HCG may also be increased in women with certain types of ovarian tumors or men with testicular tumors.

HCG, a hormone produced during pregnancy, is detectable in the blood or urine 1 to 2 days after implantation of the fertilized egg in the uterus (10 days after ovulation). It increases rapidly in the first trimester, reaching a peak 60 to 80 days after fertilization; then drops off quickly to 10-30% of the peak value for the rest of the pregnancy.

HCG maintains progesterone production by the corpus luteum in the early part of pregnancy. By the time HCG drops at the beginning of the second trimester, the placenta can make sufficient progesterone to maintain the endometrium. HCG also stimulates the development of fetal gonads and synthesis of androgens (male hormones) by the fetal testes.

• The test is negative if you are not pregnant.
• The test is positive if you are pregnant.

• ectopic pregnancy
• miscarriage
• testicular cancer
• trophoblastic tumor

• threatened abortion
• anovulatory bleeding
• first trimester pregnancy
• hydatidiform mole
• ovarian cancer
• ovarian cysts
• pelvic inflammatory disease (PID)
• polycystic ovarian disease (PCOD)

• excessive bleeding
• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin)
• infection (a slight risk any time the skin is broken)
• multiple punctures to locate veins

False positive tests may occur when certain hormones are increased, such as after menopause or when taking hormone supplements. A false positive result may also result from "phantom HCG" (when there is an abnormal interaction between the test and the patient's blood sample). This can be confirmed by a urine HCG test, which will be negative. Phantom HCG is suspected when the blood HCG test continues to be positive despite there being no pregnancy or genital tract tumor.

A pregnancy test is considered to be about 98% accurate. When the test is negative but pregnancy is still suspected, the test should be repeated in 1 week.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.