Alpha-Fetoprotein

Alpha-fetoprotein (AFP) is a globulin protein formed in the yolk sac and liver of the fetus. As the fetus develops, the level of AFP found in the mother’s serum increases.
Only minute amounts of AFP remain in the bloodstream after birth. This test is used primarily to screen for the presence of neural tube defects in the fetus, such as spina bifida and anencephaly. The test, done between 15 and 20 weeks of pregnancy, does not absolutely diagnose a birth defect. However, if the AFP is found to be abnormally high, additional testing, including ultrasonography and testing of the amniotic fluid for AFP, is needed. In many institutions, the test for AFP is now combined with measurement of estriol and human chorionic gonadotropin. This combination testing is known by various names, including “triple marker.” The measurement of these three substances provides screening for neural tube defects, trisomy 18, and trisomy 21 (Down syndrome). An accurate fetal gestational age is essential for accurate test results, because the levels of the substances all vary with gestational age. The most accurate method of assessing gestational age is ultrasonography; if unavailable, gestational age by last menstrual period is used. This testing is a screening tool; negative results do not guarantee a normal baby.
AFP is also considered a tumor marker for several types of cancer. Cancers typically are characterized by undifferentiated cells. These cells often still carry surface markers similar to those found in the fetus. The higher the AFP level, the greater amount of tumor present. Thus, AFP can also be used to assess response to cancer treatment.