What is AFP TESTING?

AFP testing is a term used to describe a grouping of tests done during pregnancy. The original reason behind this testing was to find a way to help pick up certain abnormalities in fetuses. This grouping of tests began during the 1980's with a blood test that measured the substance alpha-fetoprotein (AFP). It was found that when this substance is at a high level the fetus often has a certain type of abnormality such as spina bifida. As researchers continued to study this type of testing, they began measuring other substances so that other abnormalities might be identified. Human chorionic gonadotropin (HCG) and unconjugated estriol (UE3) can be measured along with AFP. These substances are all produced from the growing fetus and developing placenta and are found in the blood of all pregnant women. AFP can be measured in blood or amniotic fluid. HCG and UE3 can only be measured in blood.

Depending on your provider's preference, the laboratory used and the woman's personal situation, one (usually AFP) or all of these substances can be measured. They will be compared to the standard amounts that are normally present during pregnancy. If all three substances are measured, the test is referred to as a triple screen or AFP3 . If only AFP and HCG are measured, the test might be referred to as AFP+ or AFP Plus . Many providers are choosing to order the triple screen test because the results have been found to be more accurate when all three substances are measured compared to when only AFP is measured. Certain fetal conditions will make the amounts of these substances abnormal or out of balance.

A screening test tells you the likelihood or risk of a problem. It cannot give you a "yes" or "no" answer. AFP testing is designed to pick out a few women in the population who are at risk for carrying a fetus with certain problems. It is NOT a diagnostic test. A diagnostic test is very accurate (it gives you a "yes" or "no" answer) and medical decisions are made based on definite results. Since AFP testing is only a screening test, some women will be identified as being in the at risk group even though they are carrying NORMAL fetuses . If your AFP test comes back abnormal, you may be one of the women who has an abnormal result and a normal pregnancy, but were selected for further evaluation only because of the way the screening process works.

To obtain the result of your AFP test, the laboratory where your sample is sent uses a formula to calculate the result. Information and facts about you and your pregnancy are used in this formula. If the wrong information is given, then the result will be inaccurate.

The information used in the formula includes:

• How far along in the pregnancy you are when the sample is taken. This is usually stated in number of weeks gestation. If there is a question about how far along you are, an ultrasound examination will confirm your gestational weeks.
• Your weight
• Your race
• Your age
• Family history
• The presence of twins
• And, insulin dependent diabetes.

Also, if you have had any vaginal bleeding (or occult bleeding within the amniotic sac, bleeding hidden from the woman's sight) around the time the AFP blood test sample is drawn, then you are at risk for the test result being abnormal.

1. Neural Tube Defects. Neural tube defects are defects in the formation of the spinal cord and brain. It is called ANENCEPHALY when the defect is at the top of the head. Fetuses with this condition are often miscarried or die within several days of birth, as this condition is incompatible with life. SPINA BIFIDA (or myelomeningocele) happens when the defect is on the lower part of the back. In this condition, the nerves in the spinal cord do not connect properly with the nerves in the lower part of the body. This causes problems with moving lower body muscles. Complications of this condition may include infections, HYDROCEPHALUS (water on the brain), and sometimes mental retardation. Infants born with this condition usually require surgery during childhood. Neural tube defects occur in one or two of every 1,000 pregnancies.

2. Abdominal Wall Defects. Occasionally a fetus will have a defect in the muscle and skin near the "belly button." This is called OMPHALOCELE or GASTROSCHESIS and is serious but may be corrected with surgery done after the birth of the baby.

3. Chromosome Abnormalities. The most common chromosome abnormality is DOWN SYNDROME, a condition where the fetus has an excess number of chromosomes. These babies have a distinct appearance and may have several birth defects -- most commonly, abnormalities of the heart and some degree of MENTAL RETARDATION that ranges from minimal to severe. Approximately one in 750 babies born has Down Syndrome.

4. Other rarer abnormalities, such as kidney or heart problems.

First, a review of all the factors that go into the formula used to calculate the result should be done to verify accuracy. Next, the levels of the substances measured are reviewed and a risk determination is made. Then, if not already done, an ultrasound exam will confirm the woman's correct gestational age and rule out twins. If all the factors in the formula are accurate, and the test result shows the woman to be at risk for a neural tube defect, then some providers will order a second AFP test to confirm the first abnormal result. AFP testing which includes HCG and UE3 measurements -- for example, the triple screen -- is done only once. Further testing is very often recommended. If the woman is shown to be at increased risk for having a baby with Down Syndrome, then amniocentesis is most often offered.

If the woman's fetus is shown to be at risk for having a neural tube defect or an abdominal wall defect, then a special ultrasound examination called LEVEL 2 or "TARGETED" might be advised. This exam is performed by a physician with special training in ultrasonography. If the targeted ultrasound exam does not show an abnormality, then amniocentesis may be recommended. Sometimes additional targeted ultrasound examinations are recommended when all further testing shows no abnormalities. Some fetuses are at risk for growth abnormalities later in pregnancy.

Amniocentesis is the procedure used to withdraw a small amount of amniotic fluid (the water in the sac surrounding the fetus). Amniotic fluid contains cells shed from the fetus and includes copies of the genetic material the fetus possesses. Tests done on amniotic fluid yield more accurate results than tests on maternal blood. If the woman is determined to be at risk for having a baby with a neural tube defect, then, along with the level 2 (targeted) ultrasound exam, very often amniocentesis is recommended also. Approximately 98% of all open neural tube defects can be detected by testing amniotic fluid. If the fetus is at risk for having Down Syndrome, then amniocentesis will detect this with greater than 99% accuracy when this chromosome abnormality is present.

AFP testing can be done between 14 and 22 weeks along -- counted from the first day of the last menstrual period -- but is best done at between 16 and 18 weeks. If amniocentesis is planned for other reasons, then serum (blood) AFP testing will not be done. The amniotic fluid will be tested for neural tube defects and Down Syndrome. If the woman has chorionic villi sampling (CVS) early in pregnancy, which tests only the fetus' chromosomes, she can have AFP testing at 16 weeks to screen for neural tube defects. Only AFP will be measured since HCG and UE3 levels are only used to help detect chromosome abnormalities, and CVs has already tested for this. If the woman decides to have serum AFP testing, a special test request form will be filled out with information about the woman and her pregnancy. This information must be accurate to help avoid false abnormal results.\

All pregnant women should be informed of the availability of this prenatal screening test for certain birth defects. Most providers suggest and explain AFP testing, but it is not a mandatory test. Serum AFP testing is recommended for all pregnant women up to the age of 35. lt can be done after age 35, but usually amniocentesis or chorionic villi sampling is offered when the woman will be age 35 or older by the time the baby will be due. Some women are at increased risk for having a fetus with a neural tube defect. They are those who have already had a child so afflicted, those with a family history of neural tube defects and women with insulin-dependent diabetes. Women who are at increased risk for having a baby with Down Syndrome are those with a child who has Down Syndrome or who has a family history of Down Syndrome and those who are age 35 years or older.

Most women want to obtain as much information about their unborn baby as they can. Neural tube defects and Down Syndrome are major birth defects. Serum AFP testing can help identify those women who are at risk for having a baby with those problems. Prior to the advent of ultrasound and prenatal testing such as AFP testing, many abnormalities went undetected until the time of birth. The sophistication of prenatal diagnostic testing is dynamic and still evolving. Presently, every pregnant woman must ask herself how much information she wants about her unborn baby. Choosing to have this test will add to that knowledge.

The greatest disadvantage is the FALSE abnormal test result. Almost 50% of abnormal test results occur because of incorrect information supplied for the formula to calculate the result. This is why it is so important for the test request form to be filled out accurately. Approximately 40 of every 1,000 women having serum AFP testing will have a result suggesting their fetus is at risk for a neural tube defect. Of these 40 women, only one will actually have a baby with a neural tube defect. Approximately 70 of every 1,000 women having serum testing will have a result suggesting their fetus at risk for having Down Syndrome. Of these 70 women, only one or two will actually have a baby with Down Syndrome.

In summary, approximately 110 women of every 1,000 (11 of every 100, or an 11% chance) having serum testing will have an abnormal result. For those women with abnormal test results, further testing -- an ultrasound to confirm the correct gestational age on the date the blood was drawn for the test and/or amniocentesis -- will almost always confirm or rule out these abnormalities. Parental anxiety over false abnormal test results is probably our biggest concern. Serum AFP testing is an excellent prenatal screening test, but like many things, it is not perfect.

No. There is no test available that can provide an absolute guarantee. Serum AFP testing is a screening test and will detect approximately 85% of fetuses with open neural tube defects. The "triple screen" will detect 60% of fetuses with Down Syndrome in women under age 35. There are many other types of problems fetuses can have that AFP testing cannot detect. Also, there is a one in 1,000 chance that the fetus will have a neural tube defect or Down Syndrome and the AFP serum screening test result will be normal.

AFP testing is an excellent screening test designed to identify those pregnant women who are most at risk for having a baby with certain birth defects. Most women who have an abnormal screening test result will have a normal baby. Discuss the implications of having this testing with your family and healthcare provider. It is the pregnant woman's decision to proceed with this testing. Read this information carefully. If you have any questions or need clarification, please ask for it.