First trimester biochemical screening for Down syndrome: free beta hCG versus intact hCG

To compare free beta hCG versus intact hCG in first trimester Down syndrome screening we analysed 63 cases of Down syndrome and 400 unaffected control pregnancies between 10 and 13 weeks' gestation. The Down syndrome median m ultiple of the median (MoM) was significantly higher (p=0.001) for free bet a hCG (1.89 MoM) than for intact hCG (1.37 MoM). Although distributions for free beta hCG (unaffected, 0.2157; DS, 0.2322) are wider than for intact h CG (unaffected, 0.1697; DS, 0.2158), overall 27% of Down syndrome cases wer e above the 95th percentile for free beta hCG compared to 19% for intact hC G. Combined with maternal age, free beta hCG detected 45% of Down syndrome pregnancies at a 5% false positive rate. Intact hCG combined with maternal age demonstrated a detection efficiency comparable to maternal age alone (3 5% versus 32%). In contrast, a recent study (Haddow et al., 1998-NEJM 338: 955-961) indicated that intact hCG yielded a higher first trimester Down sy ndrome detection efficiency than free beta hCG (29% Versus 25% respectively ). Re-analysis of distribution parameters in the Haddow et nl. study, howev er, show that free beta hCG was actually the better marker (23% detection f or intact hCG versus 29% for free beta hCG). Copyright (C) 2000 John Wiley & Sons, Ltd.