First trimester screening for Down syndrome using free β hCG, total hCG and PAPP-A:: an exploratory study

Objective To investigate the potential utility of first trimester screening for Down syndrome using Free beta-hCG, total hCG and PAPP-A. Materials and Methods Using estimates from the literature, a simulation study was undertaken to estimate the performance of tests incorporating, Free beta-hCG, total hCG and PAPP-A at gestations of 8-12 weeks. We used sensitivity analysis to assess the effect of departures from the assumed model. Results We estimate that detection rates in excess of 75% for a false positive rate (FPR) of 3% can be achieved with first trimester measures of PAPP-A, total hCG and Free beta-hCG at 8 weeks-the addition of total hCG adding 11%. Detection rates of around 90% for a FPR of 3% can be achieved through the inclusion of nuchal translucency (NT) at 12 weeks to these early first trimester biochemical markers. Our analysis indicates that the marginal benefit of adding total hCG diminishes rapidly with gestational age and that there is little benefit from adding total hCG later than 10 weeks of gestation. Conclusion The performance of first trimester screening using early combinations of total hCG, Free beta-hCG and PAPP-A should be assessed in further studies. Copyright (C) 2007 John Wiley & Sons, Ltd.