Reactivity of amniotic fluid alpha-fetoprotein with concanavalin A-sepharose in pre-natal diagnosis of fetal malformations.

The proportion of concanavalin A-non-reactive alpha-fetoprotein was determined in 215 amniotic fluid samples from second trimester pregnancies. the median percentage for concanavalin A-non-binding alpha-fetoprotein was 35.5% at the 15th week and 32.2% at the 18th gestational week. Nineteen of the 23 pregnancies with various fetal malformations showed highly elevated total alpha-fetoprotein levels. In this group, the value for non-reactive alpha-fetoprotein was below the normal range in 12 out of 13 samples collected at 15-17 weeks of pregnancy and in four out of six samples at 18-19 weeks. Four pathological pregnancies had only moderately elevated total alpha-fetoprotein levels (5.3-7.9 SD above the mean) and two of these samples had a low percentage of the concanavalin A-non-binding fraction. The amniotic fluid alpha-fetoprotein concentration was between 3 and 5 SD and over 5 SD above the mean in four and seven normal pregnancies, respectively. The concanavalin A-fractionation classified correctly 10 out of these 11 cases. The results indicate that the determination of the proportion of concanavalin A-non-binding alpha-fetoprotein is a useful supplementary test to the total alpha-fetoprotein assay.