The performance of this test varies with maternal age and other clinical parameters. Recent research suggests that second trimester maternal serum screening of alpha-fetoprotein, unconjugated estriol, human chorionic gonadotropin and inhibin A (the Quad Screen) has 81% sensitivity for Down syndrome detection and a false-positive rate of 7%. This use is considered to be investigational. Follow-up testing is recommended for patients when the calculated second trimester Down syndrome risk is greater than or equal to 1: 270. Normal serum AFP levels for neural tube defects (NTD) are less than 2.0 multiples of the median (MoM). Normal values exclude approximately 95% of cases of anencephaly, 80% of cases of spina bifida, and 85% of ventral wall defects. Closed dysraphic states and small defects generally constitute the group in which total exclusion is not possible.