Why is the screening result for neural tube defects based on the AFP MoM value and not on the risk estimate?

With only one marker (AFP) “risk screening” is not needed so the AFP MoM value alone is used. Also, the risk estimate for neural tube defects (NTDs) depends on accurate values for the local prevalences of anencephaly and of spina bifida. These are often uncertain, for example because of differences in the use of periconceptional folic acid and because ultrasound scans may have identified anencephaly before serum testing excluding such cases from the population screened. Therefore, using the risk estimate for NTD is less accurate than it is for Down’s syndrome.