Screening for trisomies 21 and 18 in a Spanish public hospital: from the combined test to the cell-free DNA test

Objective: To describe our experience in first-trimester screening for trisomies 21 and 18 firstly by the combined test alone and secondly by cell-free (cf) DNA testing contingent on the results from a previously performed combined test. Methods: Women with singleton pregnancies attending Torrejon University Hospital in Madrid, Spain, from November 2011 to January 2016, were screened for trisomy (T)21 and T18 by the combined test at 11–13 weeks. Before the introduction of cfDNA testing, women at high risk (>1 in 250) were offered invasive testing (IT) and from January 2015 they were offered cfDNA test as well as IT. Results: Combined test was performed in 6011 pregnancies. The risk was high in 202 (3.4%) cases. There was complete follow-up for 5507 (91.6%) pregnancies. Detection rate (DR) for T21 was 83.3% (15/18) and 100% (4/4) for T18. Additionally, 2/2 (100%) cases of T13 and 2/2 (100%) cases of triploidy were also detected. False positive rate (FPR) was 3.2% (174/5488). The introduction of this contingent model was followed by a 73% reduction on the IT rate in the high-risk group, from 76.3% to 20.8%. Conclusion: Contingent screening for trisomies 21 and 18 by cfDNA testing at 11–13 weeks is feasible and has a lower IT rate than combined testing alone.