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The Screening Test

Many doctors and patients may have too much confidence in non-invasive prenatal tests, reports Beth Daley at the New England Center for Investigative Reporting.

NIPTs examine fetal DNA found within maternal blood, typically for signs of chromosomal disorders and are supposed to be better at detecting abnormalities than older blood tests and ultrasounds.

But lost among the declarations of high accuracy is that these tests are actually screens, Daley adds, and their results need to be confirmed through a diagnostic test like an amniocentesis.

For instance, she tells the New York Times Motherlode blog that tests are being marketed as 99 percent accurate, but that number refers to the accuracy of the negative results, not the accuracy of the positive results. While these new screens are better at ruling out conditions like Down syndrome than previous approaches, the rate of false positives can vary based upon maternal age and the rarity of the condition being tested.

These caveats, Daley says, are not always communicated to either doctors or patients. She further reports that some patients are seeking terminations based on screening test results alone.

In a web chat with Daley and others, Kecia Gaither, the director of perinatal outreach at Montefiore Medical Center and Albert Einstein College of Medicine, said that "a clear distinction should be made between a SCREENING test and one that is DIAGNOSTIC."

The Scan

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