What are non-invasive prenatal tests (NIPT)?
The phrase lacks specificity and, technically, there are several tests that could be categorized that way. However, NIPT is used most widely to describe genetic tests that use cell-free DNA (cfDNA) circulating in maternal blood to detect fetal trisomy 21, 18, and 13, as well as other chromosomal disorders. This is in contrast to invasive tests that require samples obtained by amniocentesis or chorionic villus sampling, procedures that are associated with a low risk of miscarriage.
Importantly, cfDNA NIPTs are screening tests, whereas invasive te sts are diagnostic. Screening tests for fetal aneuploidies aren’t new. Biochemical tests for fetal aneuploidies, such as the quadruple screen, have been performed for nearly 30 years. A key distinction between biochemical and cfDNA screening tests is the greater accuracy of the latter.
How accurate are cfDNA NIPTs?
Several large clinical studies have described impressive results. One meta-analysis reported respective diagnostic sensitivities/specificities of 99.0/99.92% (trisomy 21), 96.8/99.85% (trisomy 18), and 92.1/99.8% (trisomy 13). These high specificities are notable because they translate into very low false-positive rates (0.08, 0.15, and 0.2%, respectively). The combination of high sensitivity and a low false-positive rate has the potential to be misleading if the positive predictive value (PPV) of the test is not considered. As the proportion of true positive results divided by the number of all positive results, the PPV answers the question: What is the probability of an affected fetus given a positive result?
Importantly, the PPV of any test is dependent on the prevalence of the condition in the tested population. For the trisomies, prevalence increases along with maternal age.
For example, the pre-test risk of trisomy 21, 18, and 13 in a 35-year-old with a fetus at 10 weeks’ gestational age is 1:185, 1:470, and 1:1,500, respectively. These, combined with the performance characteristics described above, translate into PPVs of 87, 58, and 24%, respectively. As expected, the less prevalent a trisomy is in the population tested, the more likely it is that a positive cfDNA NIPT result will be falsely positive.
By comparison, the sensitivity of the best biochemical screening test for trisomy 21 is 93% at a specificity of 95%. At a trisomy 21 prevalence of 1:270, this translates into a PPV of only 6%. Clearly, cfDNA NIPTs outperform traditional biochemical tests when screening for fetal aneuploidies.
Unfortunately, this greater performance has created the perception that cfDNA NIPTs produce conclusive results and, as such, are diagnostic tests. This could not be further from the truth. Just as with a positive biochemical screening test, a positive result from cfDNA NIPT should be followed by invasive diagnostic testing.
Are cfDNA NIPTs replacing traditional biochemical screening tests?
Yes and no. Initial clinical studies evaluated cfDNA NIPT performance in women considered at high-risk of having an affected pregnancy. For that reason, professional societies have not endorsed use of the tests for other populations.
However, several studies have now demonstrated similar clinical performance in all women, regardless of pre-test risk.
In light of that, some would argue that offering biochemical screening is no longer justifiable given its low PPV. However, there are limitations to address before cfDNA NIPT can replace biochemical screening. The cost of cfDNA NIPT is high and using it as a first-line screening test is currently not cost-effective. Also, the test fails to produce a result in 1–5% of those tested, often due to a low amount of fetal DNA in the maternal blood. It is also worth noting that biochemical screening provides useful information beyond the detection of trisomies that would be lost if replaced with cfDNA NIPT. Regardless, cfDNA NIPT is here to stay, and optimal strategies for implementing it into routine care are ongoing areas of active investigation.
David Grenache, PhD, DABCC, is an associate professor of pathology at the University of Utah School of Medicine
in Salt Lake City and medical director
of the chemistry division at ARUP Laboratories. He writes about maternal-fetal laboratory medicine on his blog www.pregnancylab.net.