Karin Sundberg, Charlotte Kvist Ekelund, Susanne Kjærgaard, Helle Zingenberg, Line Rode, Caroline Borregaard Miltoft, Ann Tabor
Objectives
The primary aim was to compare the screening performance for Trisomy 21, of standard combined first trimester screening with referral to invasive testing at a cut-off at 1 in 300, with a contingent testing, consisting of referral to invasive testing at a 1 in 100 cut-off and referral to cell-free DNA (cfDNA) testing for a risk between 1 in 100 and 1 in 1000.
Methods
Singleton pregnant women with a combined first trimester risk ≥ 1 in 1000 were consecutively recruited from two Danish hospitals between August 2014 and May 2015.
First trimester combined screening was based on maternal and gestational age, nuchal translucency thickness, and levels of PAPP-A and ß-hCG. Blood samples for cfDNA testing were analysed using the Harmony Prenatal Test® giving risks for Trisomy 21, 18 and 13, and sex chromosomal aneuploidies. The cfDNA analysis was blinded from the combined risk assessment, karyotype results and pregnancy outcome. Pregnancy outcome and pre- and postnatal karyotypes were obtained from the Danish Fetal Medicine Database.
Results
Among 6449 women who had a combined first trimester screening, 869 (13.5%) had a risk ≥ 1 in 1000 and 597 were included for cfDNA testing. Among these there were 15 cases of Trisomy 21, one case of Trisomy 18 and two cases of Trisomy 13. The sensitivity for Trisomy 21 was 100% using both screening scenarios, while specificity increased significantly from 97.0% to 98.8% (p < 0.001) using the contingent approach. The sensitivity for Trisomy 21, 18 and 13 increased from 94.4% to 100% with overlapping confidence intervals, while specificity increased significantly from 97.1% to 98.9% (p < 0.0001). In seven pregnancies, the risk of a sex chromosomal aneuploidy was increased with cfDNA testing but discordant with the karyotype, corresponding to a false-positive rate of 1.2%.
Conclusions
In a clinical setting with an efficient combined first trimester screening, a contingent screening offering women with a combined first trimester risk ≥ 1 in 100 an invasive test and women with a risk from 1 in 100 to 1 in 1000 a cfDNA test had the same sensitivity for Trisomy 21, 18 and 13, but significantly increased the specificity, when compared to offering an invasive test to all women with a risk ≥ 1 in 300. Implementing a contingent screening would therefore significantly reduce the number of invasive tests at no loss of sensitivity.
