Alpha-fetoprotein was measured in 2209 amniotic fluid samples under improved assay conditions. All cases of open neural tube defects had elevated values greater than 5 SD above the mean. The false positive rate based on the 3 SD cutoff is estimated at less than 9.15%. Alpha-fetoprotein assays are recommended for all patients undergoing second trimester amniocentesis, optimally between 14 and 16 weeks' gestation.
PIP: A reevaluation of the alph-fetoprotein (AFP) assay for prenatal diagnosis of neural tube defects is presented. AFP was measured in 2209 amniotic fluid samples. All cases of open neural tube defects had elevated values greater than 5 standard deviations (SD) above the mean. The false positive rate based on the 3 SD cutoff was estimated at less than .15%. AFP assays are recommended for all patients undergoing second trimester amniocentesis.