A comparison of total and free β-HCG assays in Down syndrome screening

Free β-HCG is a new analyte that has been suggested to be superior to total HCG when used in combination with alpha-fetoprotein (AFP) for Down syndrome risk screening in early pregnancy. We have evaluated this claim on 21 samples collected from Down syndrome pregnancies and 180 samples from unaffected pregnancies. The detection rates for the combination of AFP with free β-HCG or the combination of AFP with total HCG were identical (71 per cent) but the initial screen positive rate (equivalent to the false-positive rate) was 7·5 per cent for AFP+free β-HCG screening compared with 3·5 per cent for AFP+ total HCG screening. We conclude that the case for free β-HCG is unproven and suggest that further data be collected before free β-HCG becomes acceptable.