Combined first trimester screening for trisomy 21: lack of agreement between risk calculation methods

To call attention to differences in first trimester risk estimates for trisomy 21, as calculated by two different software packages.

A total of ninety-four pregnant women who had a first trimester risk assessment for trisomy 21 that was based on maternal age, biochemical analysis and a nuchal translucency (NT) measurement. Two commonly used software packages were used for the estimation of individual risks (i.e. Wallac-Perkin-Elmer software and Fetal Medicine Foundation software).

Risk estimates derived from each software programme were strikingly different. In each case the discrepancy in reported magnitude of risk resulted from disparities between the two calculation methods for the assessment of the individual risk for trisomy 21. The disparities in risk estimates can be explained by significant differences in reported likelihood ratios for biochemical analyses (P = 0.01), NT measurements (P < 0.0001) and both screening parameters combined P = 0.003).

It is illustrated that the lack of agreement between these risk calculation methods could give rise to major counselling problems. In order to avoid confusion, there is a need for estimating individual risks of trisomy 21 in a standardized way. It is proposed to select a set of parameters that have a proven track record as judged by detection and false positive rates and then use that set exclusively, while simultaneously monitoring its performance.