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Factors Associated With Informative Redraw After an Initial No Result in Noninvasive Prenatal Testing

Benn, Peter, DSc; Valenti, Elizabeth, MS; Shah, Shailen, MD; Martin, Kimberly, MD; Demko, Zachary, PhD

doi: 10.1097/AOG.0000000000002728
Genetics: Original Research: PDF Only

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.

After a noninvasive prenatal test “no result,” the chance of an informative redraw is associated with the initial fetal fraction, maternal weight, and interval between draws.

UConn Health, Framingham, Connecticut; Natera, Inc, San Carlos, California; and Virtua Health, Voorhees, New Jersey.

Corresponding author: Zachary Demko, PhD, Natera Inc, 210 Industrial Road, Suite 410, San Carlos, CA 94070; email:

Supported by Natera, Inc, who provided the data and covered costs associated with data analysis and publication.

Financial Disclosure Dr. Benn is a paid consultant for Natera and holds stock options in the company. Ms. Valenti is an employee of Natera. Dr. Shah is on the advisory board for Natera and has received honoraria for speaking on behalf of Natera. Dr. Martin is an employee of Natera and holds stock or has options to hold stock in the company. Dr. Demko is an employee of Natera and holds stock or has options to hold stock in the company.

Each author has indicated that he or she has met the journal's requirements for authorship.

Received February 08, 2018

Received in revised form April 18, 2018

Accepted April 26, 2018

© 2018 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.