Cell-free fetal DNA (cfDNA) screening is one of the newest genetic tests on the market. It involves drawing a pregnant woman’s blood after 10 weeks gestation to detect fetal blood cells. The purpose is to screen for three chromosomal abnormalities: trisomy 13, 18, and 21. It can also identify the baby’s gender.
cfDNA screening has gotten a lot of attention because the test poses no risk to the developing baby (unlike an amniocentesis which is invasive) and it can be done so early. However, there are a few things a woman must consider before undergoing this test.
It has not been tested in the low-risk population. The companies who offer this screening tested it in a group of women who were very high-risk. For example, MaterniT21 tested its blood test in a group where 1 in 8 women were pregnant with a baby with trisomy 21, or Down’s syndrome. That is far above the prevalence seen, for example, in women who are 35 years old in their second trimester, which is approximately 1 in 200.
This matters because when you calculate the positive predictive value (PPV) in the high-risk group, it comes out to 98 percent. That means 98 percent of babies who had a test saying they have Down’s syndrome actually had it in this hypothetical high-risk group. However, in the general 35-year-old population, the MaterniT21 test only has a PPV of 63 percent. That means the test was wrong 37 percent of time!
But they say the test is 99 percent sensitive! Again, that really means nothing. The PPV is what really matters: does the baby I am pregnant with actually have the genetic disorder the test says I have? This is what we need to know. Until additional evidence is available (and studies are underway), the clinical usefulness in the low-risk population is not yet known. There have been cases of false positives reported, but since there is no centralized tracking system we don’t know how high that number is yet.
So should anyone get this test? Since it remains untested in the general population, most professional societies (including the American Congress of Obstetricians/Gynecologists and the National Society of Genetic Counselors) recommend this test only for high-risk women. That might include women older than 35 or those who have had an affected baby before or positive family history.
But doesn’t this replace an amniocentesis? No. That’s the other misconception. An abnormal cfDNA test is only an abnormal screening test. In the majority of cases, it should be confirmed by chorionic villus sampling or an amniocentesis.
The cost. This test can run upwards of $2,000 and not all insurance companies cover it. If you are considering this test, you should definitely check first.
Reviewed by Dr. Jen Lincoln, November 2018
- Morain S et al
- A new Era in Noninvasive Prenatal Testing
- N Engl J Med
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American Congress of Obstetricians/Gynecologists
- Committee Opinion # 545: Noninvasive prenatal testing for fetal aneuploidy.
- MaterniT21 Plus.
Mennuti et al
- Is it time to sound an alarm about false-positive cell-free DNA testing for fetal aneuploidy? Am J Obstet Gynecol
- 2013 Nov;209(5):415-9
- doi: 10.1016/j.ajog.2013.03.027
- Epub 2013 Mar 22.
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