Alpha-fetoprotein test



Definition

The alpha-fetoprotein (AFP) test is a blood test that is performed during pregnancy to screen the fetus for certain conditions; it is also used to screen for certain diseases in infants and children. The screening test measures the level of AFP in the mother's blood and indicates the probability that the fetus has one of several serious birth defects. The level of AFP can also be determined by analyzing a sample of amniotic fluid. This screening test cannot diagnose a specific condition; it only indicates the increase of risk for several birth defects. In infants and children, the AFP test is used to detect liver disease, certain cancerous tumors, and to monitor the progress of cancer treatment.

Purpose

Alpha-fetoprotein is a substance produced by the liver of a fetus, by tumors of the liver, by testes and ovaries, and by certain other diseases of the liver. The exact function of this protein was as of 2004 unknown. After birth, the infant's liver stops producing AFP; an adult liver contains only trace amounts. During pregnancy, the fetus excretes AFP in urine, and some of the protein crosses the fetal membranes to enter the mother's blood. The level of AFP can then be determined by analyzing a sample of the mother's blood.

By analyzing the amount of AFP found in a blood or amniotic fluid sample, doctors can determine the probability that the fetus is at risk for certain birth defects. It is very important that the doctor know precisely how old the fetus is when the test is performed, because the AFP level changes over the length of the pregnancy. AFP screening is used as an indicator of risk and then an appropriate line of testing (like amniocentesis or ultrasound) follows, based on the results.

Abnormally high AFP may indicate that the fetus has an increased risk of a neural tube defect, the most common and severe type of disorder associated with increased AFP. These types of defects include spinal column defects ( spina bifida ) and anencephaly (a severe and usually fatal brain abnormality). If the tube that becomes the brain and spinal cord does not close correctly during fetal development, AFP may leak through this abnormal opening and enter the amniotic fluid. This leakage creates abnormally high levels of AFP in amniotic fluid and in maternal blood.

Other fetal conditions that can raise AFP levels above normal include: cysts at the end of the spine, blockage in the esophagus or intestines, some liver diseases, defects in the abdominal wall, kidney or urinary tract defects or disease, and brittle bone disease.

Levels may also be high if there is too little fluid in the amniotic sac around the fetus, more than one developing fetus, or a pregnancy that is farther along than estimated. For unknown reasons, abnormally low AFP may indicate that the fetus has an increased risk of Down syndrome . Down syndrome is a condition that includes mental retardation and a distinctive physical appearance linked to an abnormality of chromosome 21 (called trisomy 21). If the maternal screening test indicates an abnormally low AFP, amniocentesis is used to diagnosis the problem. Abnormally low levels of AFP can also occur when the fetus has died or when the mother is overweight.

AFP is often part of a triple-check blood test that analyzes three substances as risk indicators of possible birth defects: AFP, estriol, and human chorionic gonadotropin (HCG). When all three substances are measured in the mother's blood, the accuracy of the test results increases. Although AFP in human blood gradually disappears after birth, it never disappears entirely. It may reappear in liver disease, or tumors of the liver, ovaries, or testicles. The AFP test is used to screen people at high risk for these conditions. After a cancerous tumor is removed, an AFP test can monitor the progress of treatment. Continued high AFP levels suggest the cancer is growing.

Description

The AFP maternal screening test is usually performed at week 16 of pregnancy. In both pregnant mothers (whose fetus is being screened) and in children, blood is drawn from a vein, usually on the inside of the elbow. For a fetus, AFP can also be measured in the sample of amniotic fluid taken at the time of amniocentesis. Test results are usually available after about one week.

Precautions

It is very important that the doctor know precisely how old the fetus is when the test is performed, because the AFP level considered normal changes over the length of the pregnancy. Errors in determining the age of the fetus lead to errors when interpreting the test results. Since an AFP test is only a screening tool, more specific tests must follow to make an accurate diagnosis. An abnormal test result does not necessarily mean that the fetus has a birth defect. The test has a high rate of abnormal results (either high or low) in order to prevent missing a fetus that has a serious condition.

Preparation

There is no specific physical preparation for the AFP test.

Aftercare

Other than making sure the bleeding stops from the needle puncture site and watching for any signs of infection at the needle site, there is no specific aftercare involved with this blood test.

Risks

The risks associated with drawing blood are minimal but may include bleeding from the puncture site, feeling faint or lightheaded after the blood is drawn, or blood accumulating under the puncture site (hematoma).

Normal results

Alpha-fetoprotein is measured in nanograms per milliliter (ng/mL) and is expressed as a probability. The probability 1:100, for example, translates into the chance that the fetus has a one in 100 chance, for example, of having the defect. An AFP level less than or equal to 50 ng/mL is considered normal.

KEY TERMS

Amniotic fluid —The liquid in the amniotic sac that cushions the fetus and regulates temperature in the placental environment. Amniotic fluid also contains fetal cells.

Fetus —In humans, the developing organism from the end of the eighth week to the moment of birth. Until the end of the eighth week the developing organism is called an embryo.

Abnormal results

The doctor inform the mother of the fetus about the specific increased risk as compared to the normal risk of a standard case. If the risk of Down syndrome is greater than the standard risk for women who are 35 years old or older (1:270), then amniocentesis is recommended. Again, the test has a high rate of showing an abnormal AFP level in order to prevent missing a fetus that has Down syndrome. This screening test only predicts risk; appropriate diagnostic testing follows an abnormal screening result. In neonatal liver disease testing, an AFP level greater than 40 ng/mL is considered abnormal. An AFP level greater than 20 ng/mL may be associated with tumors of the ovary or testes.

Parental concerns

A parent might be concerned about drawing blood from a child, but the pain from the needle puncture only lasts a moment.

When to call a doctor

If there is excess bleeding from the needle puncture site, or if hours to days later, the puncture site looks infected (red and swollen), then a doctor should be contacted.

Resources

BOOKS

Henry, John. Clinical Diagnosis and Management by Laboratory Methods , 20th ed. Philadelphia: Saunders, 2001.

Wallach, Jacques. Interpretation of Diagnostic Tests , 7th ed. Philadelphia: Lippincott, Williams, and Wilkins, 2000.

ORGANIZATIONS

March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. Web site: http://www.modimes.org.

National Cancer Institute. Building 31, Room 10A31, 31 Center Drive, MSC 2580, Bethesda, MD 20892–2580. Web site: http://www.nci.nih.gov.

Mark A. Best

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