Cell free DNA testing using the plasma of pregnant women is a noninvasive prenatal test that has tremendous potential as a screening tool for fetal aneuploidy, says a recent Committee Opinion written by the The American College of Obstetricians and Gynecologists Committee on Genetics and The Society for Maternal-Fetal Medicine Publications Committee.1
Based on previously reported evidence, the most effective screening test for aneuploidy in high-risk women seems to be cell free fetal DNA.1 Women considered to be high risk include those 35 years or older at delivery, those whose fetal ultrasonographic findings are indicative of an increased risk for anueuploidy, those who have previously had a child with a trisomy, or those with a parent with a balanced robertsonian translocation with increased risk of trisomy 13 or trisomy 21. Cell free DNA testing is also a useful follow-up tool for women who have had a positive test result in a first- or second-trimester screen.
Before the test is given, patients should be counseled about the limitations of the test. The cell free DNA test can detect only trisomy 13, trisomy 18, and trisomy 21. Although it can identify approximately 98% of cases of Down syndrome, with a false-positive rate of less than 0.5%, cell free DNA testing is not as precise as chorionic villus sampling or amniocentesis and should not replace these invasive diagnostic tests, which remain the best tests for confirming a fetal aneuploidy. Also, other than screening for the common trisomies, the cell free DNA test provides no additional genetic information. The test is also not appropriate for multiple gestations and should not be offered as a routine test or to low-risk women because there is insufficient evidence about how this test performs in these patient groups.
A cell free DNA test result that is negative does not guarantee that the fetus is unaffected by a fetal aneuploidy, nor does a positive test result confirm a trisomy. Women considered high risk who have a negative test result should be offered a second-trimester ultrasound examination to evaluate the fetus for structural anomalies. All patients with a positive cell free DNA test result should be referred for genetic counseling and offered an invasive prenatal test to confirm the diagnosis. In addition, the committees recommend that maternal serum alpha-fetoprotein screening or ultrasonographic evaluation for open fetal defects should continue to be offered to all pregnant patients.
Pertinent Points:
- Cell free DNA testing is a useful noninvasive test for patients at high risk for a fetal aneuploidy who have made an informed choice after pretest counseling.
- Physicians should not offer cell free DNA testing as part of the routine prenatal laboratory assessment.
