Maternal serum urea resistant alkaline phosphatase in Down syndrome pregnancy

The normal levels of alkaline phosphatase (AP) activity in maternal serum are virtually the same as those observed in Down syndrome (DS) pregnancies at 14-20 weeks' gestation. Using urea inhibition of AP, we observed an atypical AP isoenzyme in the neutrophils of mothers with trisomy 21 fetuses.

To assess the use of urea as a selective inhibitor of serum AP in order to seek a possible diagnostic difference between normal and DS pregnancies.

Serum AP samples from 24 DS pregnancies and 204 control cases were examined at 12-22 weeks' gestation with and without 2.5 M urea AP inhibition at 18 degrees C for 2 h. The levels of AP activity obtained without urea and the percentage urea AP inhibition were analyzed in the two groups.

Without urea treatment, no significant difference of total alkaline phosphatase activity levels was detected between the 204 normal controls and the 24 DS samples. Using 2.5 M urea AP inhibition, after 120 min of exposure at 18 degrees C, the residual activity, as a percentage of initial values of AP, showed significantly higher resistance in the DS samples (> or = 50 IU/l of total AP activity) at 15-22 weeks' gestation. However, at 12-14 weeks (< or = 45 IU/l of total AP activity), no significant difference was found between the DS and control cases.

Serum urea resistant alkaline phosphatase in DS pregnancies showed a significant difference only at 15-22 weeks' gestation, compared with normal controls.