Alpha-Fetoprotein Test Health Article

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.