Cell-free DNA screening for women at low risk for fetal aneuploidy
A high sensitivity and specificity and low false-positive rate make cell-free DNA screening an appealing draw for clinicians and patients, even for those who are at low risk for chromosome abnormalities. But a clear understanding of this test’s nuances and proper pretest and ongoing counseling are needed to answer our patients' tough questions.
In this Article
- Pros and cons of cfDNA in low-risk patients
- Optimal and less optimal candidates for cfDNA screening
- Checklist for pretest counseling for cfDNA
In one study of women found to be at increased risk based on traditional multiple marker screening, if cfDNA were chosen instead of diagnostic testing, 17% of the aneuploidies present in this group would not have been detected.18 Of all high-risk women in this study, 2%, or 1 in 50, had a chromosomal abnormality detectable by amniocentesis but not with cfDNA.
Successful tests require adequate placental DNA
Accurate interpretation of cfDNA screening also requires that an adequate quantity of placental DNA be present; this is often referred to as the “fetal fraction.” In some cases, the placental DNA volume is too low for accurate analysis, particularly in obese patients and women with specific chromosomal abnormalities.
Some laboratories measure this and do not report a result if the fetal fraction is below a specific cut-off, typically about 4%. Other laboratories do not measure or exclude cases with too little fetal DNA, raising concern that this could result in missing cases of aneuploidy. It has been noted that a placental DNA fraction of less than 8% is associated with less accurate test results, even if results are returned.8
Low fetal fraction also has been associated with maternal obesity, and in one study cfDNA failed to provide a result in 20% of women weighing more than 250 lb and 50% of women weighing more than 350 lb.19 Therefore, cfDNA is not the best option for obese women (TABLE 4).
TABLE 4 Appropriateness of cfDNA screeningin specific clinical circumstances
Optimal candidates for cfDNA screening
- High risk for trisomy based on maternal age (≥35 years)
- Ultrasound findings suggesting trisomy 13, 18, or 21
- History of prior pregnancy with trisomy 13, 18, or 21
- Positive traditional screening test
- Parental balanced Robertsonian translocation associated with risk for trisomy 13 or 21
Less optimal candidates
- Low risk for trisomy based on age and/or low risk traditional screening
- Ultrasound structural anomalies other than those specifically suggesting trisomy 13, 18, or 21
- High risk for nonchromosomal genetic disorder
- Comprehensive genetic diagnosis desired
- Maternal malignancy
- Maternal organ transplant
- Maternal sex chromosomal mosaicism or other chromosomal abnormality
- Maternal obesity
- Gestational age <10 weeks
While the free fraction is relatively constant from 10 to 22 weeks’ gestation, it is lower earlier than 10 weeks’ gestation and less likely to provide a result. For this reason, the test should not be attempted before 10 weeks’ gestation.
Recent evidence indicates that low fetal DNA fraction is associated with some chromosome abnormalities. Given this association, women with failed cfDNA results should be counseled and offered appropriate follow-up. As the association appears to be greater for trisomies 13 and 18, and triploidy, a careful ultrasound is likely to detect abnormalities in many such cases. However, it also is appropriate to offer the option of diagnostic testing, given the very high risk.
A repeat cfDNA test will be successful in some cases. Whether the patient chooses to attempt cfDNA again may depend in part on maternal body mass index (BMI), as well as gestational age—a patient at a more advanced gestation may not wish to delay obtaining definitive information given the high risk.
cfDNA screening has a low false-positive rate
One of the greatest benefits of cfDNA screening is a lower false-positive rate than is reported with traditional screening. However, when “no results” cases are also considered, the percentage of patients who require follow-up after cfDNA is close to that of traditional screening.
The chance of test failure is reported to be 0.9% to 8.1%,7,9,10 and varies in part by whether the laboratory measures fetal fraction and requires a minimum concentration.
A recent meta-analysis estimated the overall test failure rate at 3%.10 When comparing cfDNA to traditional screening, if “no results” cases are included with the “screen positive” group, the benefits of cfDNA over traditional screening are much less clear, particularly in a low-risk population.
ACOG: Offer traditional multiple-marker screening first
While multiple marker and cfDNA screening have differing performance characteristics, there are no data to support doing both tests concurrently. In fact, in a recent survey of nearly 200 women presented with different testing scenarios, women found it preferable and more reassuring to have a positive traditional screen followed by normal cfDNA results, rather that discrepant results of the 2 tests done concurrently.20
For many reasons, the approach recommended by ACOG and SMFM is to offer traditional multiple-marker screening first, and cfDNA screening or diagnostic testing as a follow-up for patients that screen positive. In that scenario, the benefits and limitations of diagnostic testing versus follow-up with cfDNA screening should be explained carefully.
