Serum Pregnancy Test
- quantitative pregnancy test, blood
- HCG quantitative serum test
- human chorionic gonadotropin level blood test
- quantitative serum beta-HCG
- quantitative beta HCG, serum
HCG (Human Chorionic Gonadotropin) is a hormone that is produced by the placenta. It appears in the blood and urine within 10 days of fertilization of the egg by the sperm (conception).
Who is a candidate for the test?
HCG is a hormone that is produced by the placenta. It appears in the blood and urine within 10 days of fertilization of the egg by the sperm (conception).
After the unborn child begins developing, he or she implants, or attaches, to the inside of the uterus or other structure inside the mother, the levels of HCG rise rapidly. The levels continue to increase throughout the first trimester (3 months) of pregnancy and reach a peak 60 to 80 days after the unborn child implants.
HCG is thought to be important in converting the normal corpus luteum into the corpus luteum of pregnancy. The corpus luteum is a hormone-secreting structure that grows on the surface of the ovary after ovulation takes place.
In pregnancy, functions of the corpus luteum include:
Because HCG is produced by the placenta, the presence of HCG in a woman's blood almost always indicates that she is pregnant. HCG is produced whether the unborn child implants in the uterus (the normal situation) or outside the uterus in the fallopian tube or on another structure in the abdomen or pelvis (an ectopic pregnancy).
An HCG test is usually done to confirm or rule out pregnancy. Women of childbearing age who have been having sex should be evaluated with some type of HCG test if they are having the following symptoms:
There are two different tests to measure HCG:
One, called a quantitative test, measures the exact level of HCG in the blood. The result can not only determine if a woman is pregnant, but also help give a rough estimate of the duration of the pregnancy. It can also help determine if the pregnancy is progressing normally. Levels that are abnormally low or high suggest the need for further evaluation and testing to determine whether an abnormal medical condition is present.
The qualitative test is not able to provide this extra information. Rather, it gives only a "yes" or "no" answer as to whether or not HCG is present. If the answer is "yes" the woman is most likely pregnant.
A version of this test can also be performed on a urine sample; however, the urine test is less sensitive and therefore does not turn positive until the pregnancy is a few weeks farther along.
How is the test performed?
A blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or "tourniquet," is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them.
A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle, and is collected in a syringe or vial. After the needle is withdrawn, the puncture site is covered with a bandage for a short time to prevent bleeding. This blood is sent to the lab to determine the amount of HCG that is circulating in the blood. HCG is measured as a certain quantity per cubic centimeter of blood.
What is involved in preparation for the test?
No special preparation is needed for this test.
What do the test results mean?
Greater than normal levels of HCG may indicate:
a tumor of the placenta with death of the unborn baby
multiple pregnancy, such as twins or triplets
ovarian cancer and other types of cancer in some cases
a normal pregnancy
blood or protein in the urine, which can interfere with the result
- use of medications to prevent seizures, drugs to treat Parkinson's syndrome, or phenothiazine drugs, such as chlorpromazine (i.e., Thorazine), by the mother
Lower-than-normal HCG levels may indicate:
a miscarriage, or abortion. The miscarriage may only be threatened and never happen, may be happening at the time of the test, or may have already occurred. Further testing can determine which of these, if any, is the case.
- use of diuretics, or "water pills," by the mother, which can interfere with the result
The test may fail to detect HCG even when a woman is pregnant if the test is done too early. It takes at least 7 days after fertilization for the test to become positive. In most cases, by the time a woman has missed her period, the test will be positive if she is pregnant. A normal pattern of HCG levels over time is expected in a healthy pregnancy.
The level of HCG increases throughout the first trimester, then gradually decreases over time. In the first trimester, a woman may have this test done repeatedly to see if the level rises normally. If it does not, it means that there is probably some difficulty with the pregnancy. After childbirth, miscarriage, or abortion, the level should quickly decrease to zero.
This test can also be measured repeatedly if there is a tumor or cancer of the placenta or ovary in some cases. This test can only be used if the tumor or cancer caused the level of HCG to be high. This can happen in some tumors that actually make HCG.
After treatment of the tumor or cancer, the level should go back down to zero. If it does not, the tumor or cancer may still be present in the body and more treatment may be needed.