The Pregnancy Lab

Should DNA-based tests for Down syndrome screening replace biochemical tests?

In a previous post I described the clinical performance of DNA-based screening tests for fetal aneuploidies like Down syndrome.  Overall, these tests have excellent detection rates (~99%) with very low false-positive rates (~0.2%).  In other words, these tests are about 99.0% sensitive and 99.8% specific.

With performance like that one might expect these to be considered diagnostic tests.  They are not! Although quite good, test results must not be interpreted as definitive evidence that a fetus does or does not have an aneuploidy.  Recent recommendations from the American College of Obstetricians and Gynecologists (ACOG) are quite clear on that issue.

In those same recommendations, ACOG also states that DNA-based screening tests may be performed only on women who are at increased risk of having a fetus with aneuloidy.  Among the indications listed for women considered to be at increase risk are:

• Maternal age 35 years or older at delivery
• Fetal ultrasound findings suggesting aneuploidy
• A previous aneuploid pregnancy
• Abnormal biochemical screening test results

The ACOG is right to avoid recommending that DNA-based screening tests are acceptable to use regardless of risk factors.  Unfortunately, many women who are not at increased risk are using these new tests as a primary screening test and that's not a good idea.

To understand why, considered a population of 100,000 pregnant women from the general population and assume that the prevalence of Down syndrome is 1 in 500 pregnancies.  That means that there would be 99,800 unaffected pregnancies and 200 pregnancies with Down syndrome.  The table below compares the results of the most commonly used biochemical screening test (the Quad test) to a DNA-based screening test.

Clearly, the DNA-based test has several advantages over the Quad test.  Its positive predictive value is nearly 17 times greater than the Quad's and a positive DNA-based test result substantially increases the odds of having an affected fetus.  So why not use the DNA-based test as a primary screening test?  For the following reasons:

• No studies have been published that have evaluated the performance of DNA-based tests in women who are not at increased risk of having a fetus with an aneuploidy
• DNA-based tests are not widely available
• The time it takes to report results of DNA-based testing is about 3 times greater than it is with biochemical testing
• DNA-based tests are considerably more expensive than biochemical tests
• Relative lack of insurance coverage for DNA-based tests

Until these these limitations can be resolved, it makes more sense to use DNA-based testing as a secondary screening test.  In other words, it is only done after one of the risk factors described by ACOG (above) are met.  Doing so greatly improves the performance of both tests (see figure below).  A limitation of this approach is that the detection rate is that of the biochemical test which is not as high as it is with the DNA-based test.  Still, given the current limitations of DNA-based testing, this 2-step testing approach makes the most sense.

DNA-based tests for Down syndrome screening show excellent clinical performance

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.

Table References: Genet Med 2011;13:913-920; Genet Med 2012;14:296-305; Obstet Gynecol 2012;119:890-901

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.