Combined first trimester nuchal translucency and second trimester biochemical screening tests among normal pregnancies

We prospectively examined whether first trimester nuchal translucency (NT) and second trimester triple test (TT) results are correlated, and determined overlapping and mutual screen-positive rates. Results of NT, TT, amniocentesis and pregnancy outcome were obtained in 508 normal pregnancies. Inter-test correlation was performed by comparing the likelihood ratios (LR). Overlapping of screen-positive cases, of NT and TT, was determined by comparing mutual risks for Down syndrome (DS) livebirth of ≤ 1: 380. Combined screen-positive rates were evaluated by using summation risk (NT and/or TT exhibiting a risk ≤ 1: 380) and calculated risk (new risk ≤ 1: 380, based on multiplication of LR(N)(T) and LR(T)(T). Screen-positive rates between NT and TT differed significantly and when either test showed an increased risk for DS, the probability of the other to predict the same was negligible (p < 0.001). Overall screen-positive rates, at a risk ≤ 1: 380, were 2% and 5.7% for NT and TT, respectively. Summation and calculated combining methods were associated with 7.5% and 2.0% screen-positive rates, respectively. Amniocentesis was performed on 20.7% of the cases, mostly screen-negative ones. Our results showed that, in normal pregnancies, NT and TT do not correlate and that their combined calculated risk in normal pregnancies is associated with a low screen-positive rate of 2.0%. Copyright (C) 2000 John Wiley and Sons, Ltd.