Objective: Noninvasive prenatal testing (NIPT) sometimes fails to provide a test result, usually as a result of low cell-free DNA fetal fraction. We investigated how initial fetal fraction, maternal weight, gestational age, and time between blood sampling contribute to obtaining an informative result when a redraw is performed.
Methods: We performed a retrospective data review of NIPT samples received between January and October 2016 by a commercial laboratory, where the initial blood draw did not yield a result and a second sample was drawn between 5 and 28 days after the initial sampling. We included cases with fetal fraction less than 2.8% (the threshold for "no result" in this laboratory) and those with higher fetal fraction but where the NIPT results could not be interpreted with high confidence.
Results: For 4,018 cases in which a redraw was recommended, a result was obtained for the second sample in 2,835 cases (70.6%) (95% CI 69.1-72.0%). For the 2,959 cases with insufficient fetal fraction, there was a result for the second sample in 1,861 cases (62.9%) (95% CI 61.1-64.6%). For this subset, the average increase in fetal fraction was 1.2% with an average interval between draws of 14 days. Informative redraw rate was strongly dependent on maternal weight and fetal fraction measured at the first draw. Gestational age was not an important determinant. Informative redraw rate increased rapidly over the first 8 days after the initial draw and more slowly thereafter.
Conclusion: Based on fetal fraction in the initial sample, maternal weight, and interval between blood draws, women can be provided with a personalized estimate of their likelihood of a result on redraw. This should aid in the counseling of women faced with the choice of reattempting NIPT, conventional screening, or an invasive diagnostic test.