Universal Cell Free DNA or Contingent Screening for Trisomy 21: Does It Make a Difference? A Comparative Study with Real Data.

The aim of the objective was to compare the detection rate for trisomy 21 of universal cell free DNA (cfDNA) screening with contingent screening. Retrospective study was carried out at 3 German centers. The study included euploid and trisomy 21 pregnancies where cfDNA and first trimester (FT) screening assessment was carried out. The FT risk for trisomy 21 was computed based on combined screening and stratified into the following classes: high risk ≥1:10, intermediate risk 1:11-1,000, low risk ≤1,001. For universal cfDNA screening, the cfDNA test results were examined. For the contingent screening model, the result of the cfDNA test was taken into account in case of an intermediate FT risk. Different strategies combining maternal age, nuchal translucency, nasal bone, beta-hCG, and PAPP-A were evaluated. Screen positivity was defined as either a high risk after FT screening or a cfDNA test indicating a high-risk result. An inconclusive cfDNA test was also considered as screen positive. The search of the database identified 2,255 euploid and 163 affected pregnancies. All affected fetuses were identified by universal cfDNA screening. 1.3% of the euploid fetuses were classified as screen positive due to final inconclusive cfDNA test result. The detection and false-positive rate of a contingent approach that is based on combined screening and cfDNA screening in the intermediate group would be 98.4% and 0.7%, respectively. With this approach, cfDNA screening would be necessary in only about 27% of all pregnancies. This study demonstrates that a contingent approach provides similar detection rates for trisomy 21 as universal cfDNA screening, by a reduction of 73% the number of cfDNA tests.

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