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
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.
- 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