How accurate is the AFP test?
Alpha-fetoprotein (AFP) is a protein produced by a developing fetus. The AFP test, usually performed at 15 to 18 weeks of pregnancy, was designed to look for babies with neural tube defects, such as an open spinal cord or brain. During this stage of pregnancy, the AFP levels of many women carrying babies with neural tube defects is higher than expected. The test will pick up the majority of babies with this condition, but unfortunately not all.
The test has a false positive rate of 5 percent. This means that 5 percent of the time, the test is positive but the baby does NOT have an open spine. If the test is elevated, and the baby's ultrasound is normal, the patient may be at risk for other complications of pregnancy, including high blood pressure or a small baby. Alternately, the baby's age may have been miscalculated or there may be an undiagnosed twin pregnancy.
The AFP test is also used to screen for Down syndrome. AFP levels tend to be lower than expected for women pregnant with a Down syndrome baby.
This AFP test is commonly used along with two other blood tests. Together, this group is called the "triple screen." It's not a perfect test, but alerts the doctor to a potentially high-risk pregnancy. If the results are positive, more evaluation and further testing can be performed.
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