Prenatal diagnosis of neural tube defects. 1. Problems and pitfalls: analysis of 2495 cases using the alpha-fetoprotein assay.

Experience with the analyses of 2495 cases of neural tube defects (NTDs) is reviewed and the lessons learned from the use of alpha fetoprotein (AFP) assay as a diagnostic tool are shared. Of the 2495 consecutive cases 49 NTDs were diagnosed with AFP levels 3 standard deviations above the mean. As expected closed NTD did not have an elevated AFP level. False positive AFP elevations were observed in 122 of the 2495 cases. Only 1 false negative was found. AFP assay of amniotic fluid froM 1858 patients without a family history of NTD and studied primarily for fetal karyotyping yielded a freqeuncy of 1 NTD in 310 cases. It is concluded that any amniotic fluid study in the second trimester of pregnancy should include an AFP assay.