The use of biochemical screening tests to identify pregnant women who are at high risk of having a fetus with Down syndrome is well established. Biochemical screening began nearly 30 years ago and, over the years, the tests have evolved and improved. Now there’s a new kid on the screening test block and it’s name is DNA.
The discovery of cell-free fetal DNA in maternal plasma in 1997 opened up new possibilities for Down syndrome and other aneuploidy screening protocols. Rather than rely on biochemical testing to determine a biochemical phenotype, DNA-based tests have been developed that can detect the molecular pathology of aneuploidies (e.g. a fetus that has more than the expected 2 copies of chromosomes 21, 18, or 13; the cause of Down syndrome, Edwards syndrome, and Patau syndrome, respectively).
We’ve written about DNA-based screening tests before (here and here) and described the clinical performance of the Sequenom test. Now, other clinical performance studies have been published for 3 of the 4 tests that are (or will be) commercially available. As expected, all of them show excellent clinical performance. As shown in the table below, the detection rates for trisomy 21 are greater than or equal to 99% with very low false-positive results. Similar performance has been reported for trisomy 18 and 13.
By comparison, the detection rate of the best biochemical Down syndrome screening test (the Integrated test) is very good at 93%. However, about 5% of all Integrated test results are false-positive. A 5% false-positive rate may not seem very high but it is. For example, consider a population of 100,000 pregnant women who choose Integrated testing in the second trimester. The prevalence of Down syndrome in the second trimester is about 1 in 500 pregnancies so 200 of those 100,000 women will have a fetus with Down syndrome and 99,800 women (100,000 – 200) will have unaffected fetuses. Of those 99,800 women with unaffected fetuses, 4,900 will have a false-positive Integrated test result.
Because the false-positive rate of the DNA-based tests is so low (about <0.2%), then if those same 100,000 women were screened there would be only 200 false-positive results, a 96% decrease!
Does this mean that DNA-based tests should replace biochemical screening tests? Probably not but I’ll leave the explanation as to why for my next post.