Cell-free DNA Testing in Routine Practice: Characterisation of a Cohort with Positive Results for Trisomies, Sex Chromosome Anomalies and Microdeletions.

Cell-free DNA (cfDNA) testing is increasingly used as a screening method not only for trisomy (T) 21 but also for T18 and T13, sex chromosome anomalies (SCA) and microdeletions. Based on cases with a positive cfDNA result in our specialised prenatal practice, this study aims to characterise the usage of cfDNA testing and to estimate the positive predictive value (PPV) in routine practice in Germany. In this retrospective study we analysed the data of all pregnant women with a positive cfDNA result seen between 09/2013 and 12/2019. Women were either referred due to the positive result or the test was initiated in our practice. The primary parameter of interest was the concordance of cfDNA tests with confirmatory genetic testing. We encountered 81 cases with a positive cfDNA test (T21: 49.4%; T18: 9.9%; T13: 8.6%; SCA: 22.2%; 22q12del: 8.6%). The PPV was 95.0% for T21, but considerably lower for T18 (55.6%) and T13 (28.6%). For SCAs it was 23.1% and no case with DiGeorge syndrome was confirmed. 63% of the patients had not received a fetal anomaly scan before cfDNA testing. In first-trimester fetuses with a cfDNA test predicting an autosomal aneuploidy, fetal anomalies were detected in 90.3% of the cases. No false positive case had an abnormal US result. Despite the excellent specificity of cfDNA tests, the PPV for aneuploidies other than T21 is low in routine practice. In discordance with the current guidelines, cfDNA test is often used without a previous detailed anomaly scan. Our data provide valuable information to assist patient counselling and shared decision making.