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serum pregnancy test Alternative Names
quantitative pregnancy test, blood, HCG quantitative serum test, human chorionic gonadotropin level blood test, quantitative serum beta-HCG, quantitative beta HCG, serum
Definition
This is a test that measures the exact amount of the pregnancy hormone, human chorionic gonadotropin (HCG), in the bloodstream.
Who is a candidate for the test?
HCG is a hormone that is produced by the placenta. It is detectable in the blood and urine within 10 days of fertilisation. After the fertilised egg 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 of pregnancy and reach a peak 60 to 80 days after the fertilised egg 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: - promoting the growth of the foetus through the 12th week of pregnancy.
- stimulating the development of foetal sex organs.
- promoting the synthesis of male hormones by the foetal testes. Interestingly, an elevated level of HCG in men may indicate the presence of a testicular tumour.
Because HCG is produced by the placenta, the presence of HCG in a woman's blood indicates that she is most probably pregnant. HCG is produced regardless of the site of implantation of the egg in pregnancy. Usually this occurs within the uterus, but may happen outside the uterus, which is called an ectopic pregnancy. HCG may also be a sign of other conditions discussed below.
This test may be done to confirm or rule out pregnancy. Women of childbearing age who have been having sex should be screened with some type of HCG test if they are having the following symptoms: - delayed menstrual period
- breast tenderness
- pelvic pain
- irregular spotting or bleeding
- nausea or vomiting
There are two different tests to measure HCG. The other test is called a qualitative test. This can only give a "yes" or "no" answer as to whether or not HCG is present. This test is part of most home pregnancy test kits. The quantitative test gives a "yes" or "no" answer and gives the exact level of HCG in the blood. The quantitative beta HCG measurement can help give a rough estimate of the age of the foetus. It can also help determine if the pregnancy is progressing normally. Levels that are abnormally low or high may be signs that an abnormal medical condition is present. This would suggest the need for further evaluation and testing. The qualitative test is not able to provide this extra information.
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 laboratory to determine the amount of HCG that is circulating in the blood. HCG is measured as a certain quantity per cubic centimetre 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 tumour of the placenta with death of the foetus
- 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, by the mother
Lower-than-normal HCG levels may indicate: - ectopic pregnancy
- 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 fertilisation 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, and 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 doesn't, the pregnancy is often considered in trouble. After childbirth, miscarriage, or abortion, the level should quickly decrease to zero.
This test can also be measured repeatedly if there is a tumour or cancer of the placenta or ovary in some cases. This test can only be used if the tumour or cancer caused the level of HCG to be high. This can happen in some tumours that actually make HCG. After treatment of the tumour or cancer, the level should go back down to zero. If it doesn't, the tumour or cancer may still be present in the body and more treatment may be needed.
Author: Eva Martin, MD
Reviewer: eknowhow Medical Review Panel
Editor: Dr John Hearne
Last Updated: 26/05/2005
Contributors
Potential conflict of interest information for reviewers available on request |
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