Heat-stable and immunoreactive placental alkaline phosphatase in maternal serum from Down's syndrome and trisomy 18 pregnancies.

Placental alkaline phosphatase (ALP) activity was investigated in second-trimester maternal sera from 37 pregnancies with Down's syndrome, 28 pregnancies with trisomy 18, and in a series of 497 controls using a fluorimetric heat inactivation assay and specific immunoassay. After conversion of individual analyte values to multiples of the normal gestational median (MOM), no significant differences in total or placental ALP activities were found in the trisomy 21 or trisomy 18 cases (P > 0.01). In the Down's syndrome pregnancies, total ALP activity was 0.93 MOM, heat-stable ALP activity was 1.09 MOM, and placental ALP (by immunoassay) 0.96 MOM. In the trisomy 18 cases, total ALP activity was 0.90 MOM, heat-stable ALP activity was 0.79 MOM, and placental ALP (by immunoassay) 0.94 MOM. We conclude that neither total nor placental ALP activity is a useful marker for Down's syndrome or trisomy 18 screening.