How likely a positive cfDNA result is to be right
It depends on your age and on which condition.
Here is what the studies actually found for a positive result for trisomy 21.
| Study and population | Chance a positive result is real |
|---|---|
| NEXT trial, 15,841 women, general population, mean age 30.7 | 80.9% |
| Same trial, standard screening instead of cfDNA | 3.4% |
| CARE study, 1,914 women, routine care, mean age 29.6 | 45.5% |
| SMART study, low-risk group | 85.7% |
| SMART study, high-risk group | 97.5% |
For trisomy 18 and 13 the predictive value is lower. In the SMART study a positive trisomy 18 result was correct 50.0% of the time in the low-risk group and 81.3% in the high-risk group. For trisomy 13 it was 62.5% and 83.3%.
Read the third rowIn the CARE study, fewer than half of positive trisomy 21 results were real. And notice the second row: with older-style screening, about 3 in 100 positives were real, which means 97 women in 100 were told they had a positive screen for nothing.
The mirror image of this questionThis topic is about how often a positive result is wrong. Part 4 asks the other question, which is what a normal result actually rules out, and how much of a baby these tests never examine at all. See Part 4, The perfect baby.
Where this comes from:
146. Norton ME, Jacobsson B, Swamy GK, Laurent LC, Ranzini AC, Brar H, et al. Cell-free DNA analysis for noninvasive examination of trisomy. N Engl J Med. 2015;372(17):1589-1597.
147. Bianchi DW, Parker RL, Wentworth J, Madankumar R, Saffer C, Das AF, et al. DNA sequencing versus standard prenatal aneuploidy screening. N Engl J Med. 2014;370(9):799-808.
148. Dar P, Jacobsson B, MacPherson C, Egbert M, Malone F, Wapner RJ, et al. Cell-free DNA screening for trisomies 21, 18, and 13 in pregnancies at low and high risk for aneuploidy with genetic confirmation. Am J Obstet Gynecol. 2022;227(2):259.e1-259.e14.
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