COMBINING INHIBIN-A WITH EXISTING 2ND-TRIMESTER MARKERS IN MATERNAL SERUM SCREENING FOR DOWNS-SYNDROME

To assess the value of inhibin A as an additional second-trimester mat ernal serum marker of Down's syndrome we studied 56 affected and 280 u naffected pregnancies matched for gestational age. The median level in the cases was 1.62 multiples of the gestation-specific median (MOM) i n the controls, with 95 per cent confidence limits of 1.34-1.96. The d istribution of inhibin levels in affected and unaffected pregnancies w as approximately log Gaussian, with means about 1 standard deviation a part. This degree of separation was similar to that for human chorioni c gonadotropin (hCG), free beta-hCG, and unconjugated oestriol (uE(3)) , but about double that of alpha-fetoprotein (AFP) measured in the sam e samples. Inhibin was largely uncorrelated with AFP and uE(3), wherea s the log correlation coefficient with hCG was 0.29 (P=0.19) for Down' s syndrome and 0.41 (P<0.0001) for unaffected pregnancies; with free b eta-hCG, it was 0.18 (P=0.38) and 0.38 (P<0.0001), respectively. On th e basis of these results and other pubished studies, we estimate that measuring inhibin A in addition to AFP and hCG or free beta-hCG (with or without uE(3)) will increase the detection rate for a fixed 5 per c ent false-positive rate by about 7 per cent.