Normative levels and accuracy of the test are dependent on factors such as maternal age, race, weight and gestational age. A complete medical history must accompany the sample to allow accurate interpretation of the results. It should include such information as date of last menstrual period, estimated date of delivery, date of ultrasound, if the mother is an insulin-dependent diabetic and if there is a family history of Down’s syndrome, chromosomal abnormalities or neural tube defects.
MSAFP blood levels fall in three ranges: low, normal or elevated. An elevated MSAFP level -- 2 1/2 times above normal -- is associated with open neural tube defects (spina bifida) and central nervous system defects (anencephaly). Elevated levels are also associated with twins and an open anterior abdominal wall defect of the fetus. Elevated levels may also play a role in late-pregnancy complications such as preterm delivery, intrauterine growth retardation and hemorrhage. Abnormally low MSAFP levels are associated with Down's syndrome and other chromosomal abnormalities. However, a high or low result does not necessarily mean that the baby has any of those abnormalities.
Neural tube defects are detected in approximately one out of every 1,000 to 1,500 pregnancies. A positive test result will occur in one to two percent of all women undergoing AFP testing. The AFP test identifies 80 to 90 percent of fetuses with spina bifida, anencephaly or abnormality of the abdominal wall. For pregnant women age 35, the risk of Down’s syndrome is one in every 270 pregnancies, increasing in risk with maternal age.