pregnancy.
Cell-free DNA screening does not assess risk of fetal
anomalies such as neural tube defects or ventral wall defects; patients who are undergoing cell-free DNA screening should be offered maternal serum alpha-fetoprotein screening or ultrasound evaluation for risk assessment.
Patients may decline all screening or diagnostic test-ing for aneuploidy.
Introduction
Noninvasive prenatal screening that uses cell-free DNA from the plasma of pregnant women offers tremen-dous potential as a screening method for fetal aneu-ploidy. In 2011, cell-free DNA analysis became clinically available and the American College of Obstetricians and Gynecologists and the Society for Maternal–Fetal Medicine recommended it as a screening option for women at increased risk of fetal aneuploidy. This popula-tion was defined as women 35 years or older, fetuses with ultrasonographic findings indicative of an increased risk of aneuploidy, women with a history of trisomy-affected offspring, a parent carrying a balanced robertsonian trans-location with an increased risk of trisomy 13 or trisomy 21, and women with positive first-trimester or second-trimester screening test results. Given the increasing data available on its use as a screening test in the general obstet-ric population, this document was updated to review the advantages and limitations of the application of cell-free DNA screening in all pregnant women.
Circulating cell-free DNA of fetal origin comprises approximately 3–13% of the total cell-free maternal DNA after 10 weeks of gestation and is thought to be derived primarily from the placenta. The cell-free DNA test will screen for only the common trisomies and, if requested, sex chromosome composition. Testing can be performed starting as early as 9 weeks and until delivery. A number of laboratories have validated different techniques for the use of cell-free DNA as a screening test for fetal aneuploidy; all of the data rely on next-generation sequencing technolo-
gies and advanced bioinformatic analyses (1–7). All tests have a high sensitivity and specificity for trisomy 18 and trisomy 21, regardless of which molecular technique is used (Table 1). Sensitivities for trisomy 13 and sex chro-mosome abnormalities are somewhat lower, averaging 80–90%, but the specificity remains greater than 99% for each condition. Accuracy of sex determination generally exceeds 98% (1, 4–18). Regardless of which technology is used, results typically are available within 7–10 days of maternal sampling. The specificity for each screened con-dition usually is reported separately, so false-positive rates are cumulative and may approach 1%.
Laboratories report cell-free DNA test results in various ways. Some laboratories report aneuploidy risk as either “positive” or “negative,” whereas others report the chance of aneuploidy. The laboratories that report the chance of aneuploidy most commonly use “>99%” as indicative of high risk and “<1/10,000” as indicative of low risk, although more intermediate results occasion-ally are reported. Neither of these reporting methods is as useful to obstetric providers and patients as a positive predictive value (the chance that the positive test result is a true positive) (see Table 1) or a residual risk (the chance that a negative test result is false). Given the importance of these data in providing accurate and understandable information to patients regarding screening test results, the American College of Obstetricians and Gynecologists and the Society for Maternal–Fetal Medicine encourage all laboratories to report results with positive predic-tive values and residual risk values for each aneuploidy tested.
The fetal fraction, the amount of the cell-free DNA in the maternal blood that is of fetal origin, is essential for accurate test results. Some laboratories require a fetal fraction of at least 4% for a reportable result. Other laboratories, however, do not measure or report the fetal fraction. The fetal fraction typically increases with advancing gestational age. Overall, the chance of screen failure ranges from approximately 1% to 8% and varies depending on the laboratory and the methodology used (1, 4, 5, 14). Results may not be obtained because of low fetal fraction in patients carrying aneuploid fetuses or those who are obese. For patients weighing more than 250 pounds, 10% or more may have a fetal fraction of less than 4% (19). Rates of aneuploidy as high as 23% (due to low fetal fraction or other unknown factors) have been reported for women who fail to receive an interpretable result from cell-free DNA testing. Women whose results are not reported, indeterminate, or uninterpretable (a “no call” test result) from cell-free DNA screening should receive further genetic counseling and be offered com-prehensive ultrasound evaluation and diagnostic testing because of an increased risk of aneuploidy. Although repeat screening can be performed, it may delay the diagnosis of aneuploidy, potentially limiting reproductive options, and only 50–60% of repeat screens will provide a result (14, 20).