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 frequently used for determining weight correction factors, but recently reciprocal-linear regression has been suggested to have advantages. We compared both methods of weight correction using data from two screening programmes carried out by this laboratory, one using alpha-fetoprotein (AFP) and total human chorionic gonadotrophin (HCG) (n = 129,143) and the other, AFP and free beta-HCG (n = 39,982). The reciprocal-linear method fitted the data more closely but did not significantly alter the detection rate or screen positive rate (SPR) for Down's syndrome or NTD with either dataset. Without correction, women heavier or lighter than average weight had significantly different SPRs for Down's syndrome and NTD compared with those weighing close to the median weight. Both correction methods smoothed out the variability in the SPR for Down's syndrome to a similar degree, but reciprocal-linear regression was much better at reducing the variability in SPR for NTD and its use is therefore worthwhile.