Evaluation of different weight correction methods for antenatal serum screening using data from two multi-centre programmes

Weight correction of serum markers is widely used when screening for Down's syndrome and open neural tube defects (NTD) because marker concentrations decrease with increasing maternal weight. Log-linear regression is frequent ly used for determining weight correction factors, but recently reciprocal- linear regression has been suggested to have advantages. We compared both m ethods of weight correction using data from two screening programmes carrie d out by this laboratory, one using alpha-fetoprotein (AFP) and total human chorionic gonadotrophin (HCG) (n = 129143) and the other, AFP and free bet a-HCG (n = 39982). The reciprocal-linear method fitted the data more closel y but did not significantly alter the detection rate or screen positive rat e (SPR) for Down's syndrome or NTD with either dataset. Without correction, women heavier or lighter than average weight had significantly different S PRs for Down's syndrome and NTD compared with those weighing close to the m edian weight. Both correction methods smoothed out the variability in the S PR for Down's syndrome to a similar degree, but reciprocal-linear regressio n was much better at reducing the variability in SPR for NTD and its use is therefore worthwhile.