Purpose: First trimester screening (FTS) according to Nicolaides is now a worldwide established method for prenatal aneuloidy screening. An improvement was achieved by the "Advanced First Trimester Screening" (AFS). It was the aim of the current study to set up scatter plots from nuchal translucency (NT), Papp-A and fbeta-hCG, to derive likelihood ratios therefrom and to apply them to a test collective. We wanted to examine whether or nor the test performance of FTS could further be improved.
Material and methods: In a multicentre study 10 136 singleton pregnancies were recruited. Risk assessment for the presence of aneuploidies was performed by the Pia Fetal Database (PIA). In addition, all data were recalculated by the AFS module of the online platform www.firsttrimester.net . In a third step, a newly developed algorithm was utilised, in which the foetal parameters DeltaNT, Papp-A and fbeta-hCG were put into a three-dimensional scatter plot. The surrounding volume was segmentally divided and for each space the relation of the healthy to the affected foetuses found therein was determined. Furthermore, for all participiants of this study, the likelihood ratios were examinied at the segment according to the currently measured values. This method is designated as AFS-3D.
Results: Within the observed fetuses, 86 cases with aneuploidy were detected. By appropriate choice of the cut-off the sensitivities were found to be 83 % (PIA and AFS) and 82 % (AFS-3D), respectively, and do not differ significantly from each other. The specificity could be improved from 94 % (PIA) to 96 % (AFS) and was further advanced to 98 % by the AFS-3D method. At the same time the false positive rate was lowered from 654 (PIA) to 397 (AFS) and 228 (AFS-3D) cases, respectively.
Discussion: By means of the new AFS-3D method the same count of diseased fetuses was detected compared with prior screening tests. Simultaneously, expectant mothers were spared from unnecessary invasive diagnostics in 65 % of the cases. The choice of an altered cut-off or other volume shapes are feasible and should be examined in further studies.