Age-independent indices in second-trimester serum screening for Down's syndrome are useless

Serum screening for fetal Down's syndrome in the second trimester is far more efficient in older mothers than in younger ones because the risk calculation is based on maternal age. In recent publications, several authors have tried to better the 'classical' test for Down's syndrome screening by creating an age-independent index. This procedure, however, requires the assumption that at least for one parameter the mean MOM or the variance is not constant in relation to maternal age, leading to a smaller overlap in younger women. This study, based on almost 3000 normal cases and 161 cases with Down's syndrome, could not demonstrate any relationship between maternal age and MOM distribution for alpha-fetoprotein, unconjugated oestriol, total human chorionic gonadotropin (hCG), and free beta hCG. We conclude that for a constant false-positive rate, the detection rate of the 'classical' test for Down's syndrome screening in younger women cannot be bettered by creating an age-independent index.