Choice between using delta‐NT or NT MoM

Spencer asks a question in the title to his paper, namely ‘Delta-NT or NT MoM: which is the most appropriate method for calculating accurate patient-specific risks for trisomy 21 in the first trimester?’1. We have read the paper, and to understand it better we would be grateful if the authors would clarify the calculation of likelihood ratios and present their observed centiles of nuchal translucency (NT) (in millimeters) in affected and unaffected pregnancies. Confirm that the likelihood ratios according to delta-NT measurements were determined empirically? If so, provide the numbers of Down syndrome and unaffected pregnancies that contributed to each likelihood ratio estimate and what were the likelihood ratios for each for a given delta-NT category? This could be done by presenting Figure 11 as a table. Was an equation for the association shown in Figure 11 used? If so, what was it? If not, how were likelihood ratios determined between group means of delta-NT? Since in antenatal screening for Down syndrome NT is used with other screening markers, it is necessary to know whether the authors assume that likelihood ratios derived from delta-NT values are not correlated with the other markers. If associations are taken into account in the calculation, what are the relevant correlation coefficients and how are they used in the calculation? Produce a table showing for Down syndrome and unaffected pregnancies the median NT in millimeters and 5th and 95th centiles (calculated by counting) in the following groups of gestational age: ≤ 80, 81–83, 84–86, 87–89, 90–93 and ≥ 94 days. These groups ensure that there would be at least 30 Down syndrome cases in each group. The above details would enable other groups to replicate the methodology and calculate the risk of having a Down syndrome pregnancy from different NT values either alone or in combination with other markers. It would also permit a direct comparison between the two methods. A. R. Rudnicka*, N. J.