HEAT-STABLE AND IMMUNOREACTIVE PLACENTAL ALKALINE-PHOSPHATASE IN MATERNAL SERUM FROM DOWNS-SYNDROME AND TRISOMY-18 PREGNANCIES

Placental alkaline phosphatase (ALP) activity was investigated in seco nd-trimester maternal sera from 37 pregnancies with Down's syndrome, 2 8 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 ges tational median (MOM), no significant differences in total or placenta l 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 immu noassay) 0.96 MOM. In the trisomy Is cases, total ALP activity was 0.9 0 MOM, heat-stable ALP activity was 0.79 MOM, and placental ALP (by im munoassay) 0.94 MOM. We conclude that neither total nor placental ALP activity is a useful marker for Down's syndrome or trisomy 18 screenin g.