Women concerned about the health of their babies have several options for screening and diagnostic tests. One common screening test is the AFP test, also called multiple marker screening or quadruple screening. This test, done at 15 to 20 weeks gestation, measures the levels of alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), estriol (uE3), and Inhibin A. Depending on the levels of the chemicals, the test can indicate whether a pregnancy is at increased risk for Down syndrome, trisomy 18, neural tube defects and sometimes other conditions.
When It Is Done
Blood for AFP screening should be drawn at 15 to 20 weeks of pregnancy. Sometimes AFP screening is combined with other tests done at 10 to 14 weeks to yield a more comprehensive result (called Integrated Screening). Knowing the exact gestational age of the baby is critical, since the chemical levels change during pregnancy. False positive results often occur when the pregnancy was thought to be earlier or later in gestation than it actually is.
Conditions Screened
The AFP test typically screens for at least three conditions: Down syndrome, trisomy 18 and neural tube defects. Down syndrome and trisomy 18 are both chromosome conditions. Down syndrome is caused by an extra chromosome number 21 and is associated with mental retardation, characteristic facial features and increased risks for other health problems. Trisomy 18 is more severe; most babies die in the prenatal or newborn period from heart defects and brain abnormalities. Neural tube defects cause openings in the spine or brain of the baby, resulting in neurological problems or death. Depending on which laboratory offers the test, AFP screening may also test for other conditions.
Accuracy
Detection rates for Down syndrome and trisomy 18 vary according to the mother's age; the test detects a higher proportion of cases of these two conditions as the mother gets older. However, the screen-positive rate, or the proportion of women who receive a positive test result, also increases with the mother's age. Overall, AFP screening detects about 80 percent of cases of Down syndrome, 67 percent of cases of trisomy 18, and 80 to 97 percent of cases of neural tube defects.
Advantages
When women are properly counseled about the benefits and limitations of the AFP test, it can be useful to help assess the level of risk for specific conditions in the baby. A negative test result can provide reassurance that the baby is probably healthy. A woman may choose for forgo invasive testing, which carries a risk of miscarriage, if the result is negative. A positive test result suggests the possibility of a problem, indicating that further assessment is needed.
Drawbacks
The main drawback of the AFP test is the fact that it is not conclusive. A positive screening test result must be followed by additional tests, such as ultrasound and amniocentesis, to determine whether the baby does indeed have a problem. Anxiety often runs high following a positive test result. Melissa Trant, M.S., a genetic counselor, encourages her clients to keep their AFP results in perspective: "There are many reasons for a positive AFP test result. Discussion of the results and consideration of additional tests is certainly warranted, but in most cases, the baby is healthy." On the other hand, negative test results may lead to false reassurance as a pregnancy with a negative AFP test result may still be born with an anomaly.
References
- PregnancyToday: Pregnancy Checkups & Tests - AFP
- Melissa Trant, M.S., CGC; Genzyme Genetics; Los Angeles
- "Clinical and Experimental Obstetrics & Gynecology"; Mid-trimester Maternal Serum Markers in Predicting Adverse Pregnancy Outcome; G Androutsopoulos et al; Sept. 2009
