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We agree with Drs Perales-Marín, Morales-Rosello and Quiroga that the best approach for widespread introduction of cell-free DNA (cfDNA) testing is to offer the test contingent on the results of the first-trimester combined test (maternal age, fetal nuchal translucency thickness and serum free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A levels). We also agree that the primary aim of cfDNA testing should be detection of trisomy 21, rather than trisomies 18 and 13 which can be identified easily by the combined test. We have estimated previously1 that the respective detection rate (DR) and false-positive rate (FPR) in screening for trisomy 21 by the combined test are 85% and 2.1% at a risk cut-off at the time of screening of 1 in 100, 93% and 7.2% at a risk cut-off of 1 in 500, 95% and 11.9% at a risk cut-off of 1 in 1000, 97% and 19.0% at a risk cut-off of 1 in 2000 and 98% and 24.3% at a risk cut-off of 1 in 3000. As with any screening test, increased DR is inevitably achieved at the expense of increased FPR. The UK National Screening Committee has recommended that the cfDNA test should be offered to women with a risk ≥ 1 in 150 at term, which is equivalent to a risk of 1 in 100 at the time of first-trimester screening2; we disagree with this policy, because this means that the cfDNA test would be offered to only about 2% of the population and the overall DR of trisomy 21 would be, at best, only 85%. The cost of the cfDNA test is decreasing and this trend will be accelerated by its rapidly increasing uptake. We agree with Drs Perales-Marín, Morales-Rosello and Quiroga that, in a national program, a more reasonable risk cut-off for offering the cfDNA test is 1 in 500, whereby the test would be carried out in < 10% of the population, with a substantial increase in DR in comparison to that currently achievable. M. M. Gil†‡, R. Revello†, L. C. Poon†, R. Akolekar†§ and K. H. Nicolaides*† †Harris Birthright Research Centre for Fetal Medicine, King's College Hospital, London, UK; ‡Obstetrics and Gynecology Department, Torrejon University Hospital, Torrejón de Ardoz, Madrid, Spain; §Department of Fetal Medicine, Medway Maritime Hospital, Gillingham, Kent, UK *Correspondence. (e-mail: mmar1984@hotmail.com)

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