Low levels of amniotic fluid placental alkaline phosphatase in Down's syndrome

OBJECTIVE: To investigate amniotic fluid placental alkaline phosphatase (PLAP) levels in normal and trisomy 21 pregnancies. DESIGN: Cross sectional study. SETTING: A tertiary referral prenatal diagnostic service. SUBJECTS: Three hundred and eleven women with singleton pregnancies of normal karyotype between 10 and 23 weeks gestation and 31 women with pregnancies associated with trisomy 21 Down's syndrome. OUTCOME MEASURES: PLAP levels were measured by immunoradiometric assay in amniotic fluid obtained by amniocentesis. RESULTS: Amniotic fluid PLAP was detectable from 12 weeks gestation and the median value rose to a peak of 4.57 iu/l at 18 weeks. Pregnancies associated with Down's syndrome had significantly lower levels with a median multiple of median (MoM) of 0.638, (U = 3374, P = 0.0016, 95% CI = 0.50, 0.89). For the 20 women with trisomy 21 pregnancies detected at 16 to 18 weeks, the median MoM was 0.482, (U = 3694, P = 0.0011, 95% CI = 0.37, 0.85). CONCLUSION: These data demonstrates that PLAP levels are reduced in the amniotic fluid of women carrying a fetus with trisomy 21.