![]() Although this is likely a rare occurrence since experts always recommend a true diagnostic test to follow up a positive screening result, an investigation by the New England Center for Investigative Reporting stated that three women at Stanford University had aborted healthy pregnancies after receiving high-risk results from the screens. Gammill doesn’t know any women who terminated their pregnancies based solely on the results from prenatal screens, nor has she heard of any from other doctors she knows. “It can be just an incredible amount of stress on those patients.” “Patients may say, ‘I chose to do this test because of the test performance and now I’m faced with a positive, and where does that leave me?’” Gammill said. New tests are coming on the market so quickly that even doctors and genetic counselors are challenged to keep up with all of the changes. Unclear resultsĪlthough these blood tests are meant for screening purposes only, meaning they cannot definitively diagnose a disease or disorder but rather only point to the likelihood the condition exists, the test manufacturers market the products directly to patients and often don’t clearly explain their drawbacks, touting instead the most impressive sounding of the tests’ statistics.Īnd most patients don’t understand those statistics, or the difference between screening and diagnostic tests, such as amniocentesis and chorionic villus sampling, which can definitively reveal a condition. The investigators hope their results will spur researchers and the test manufacturers to study larger numbers of women to answer that question. While the researchers still don’t know exactly how often these particular chromosome duplications influence test results, they used computational models to show that the phenomenon might underlie a substantial fraction of these tests’ false positives. One of the women’s duplications was so large that it effectively increased her probability of getting a false positive on the test to 50 percent, the authors wrote.Īll four women went on to have healthy babies. They found that two of these women indeed had extra DNA on one of their own copies of the chromosome. In their study, Shendure, Gammill and their colleagues looked at four women who’d received false positive results of a trisomy for chromosome 18. In fact, some people can have more than the “normal” amount of DNA and never know it, due to harmless duplications of pieces of their chromosomes. That analysis is predicated on a simple but flawed assumption, the authors wrote - that every woman has the same amount of DNA. Trisomies of chromosomes 13, 18 and 21 are by far the most common (other trisomies nearly always result in very early miscarriage). To identify too many or too few chromosomes, the tests ask whether there’s the right proportion of genetic material coming from the chromosomes most likely to be altered in a genetic condition. But moms have cell-free DNA in their blood too, mixed with and often indistinguishable from their babies’ (after all, half of baby’s DNA is identical to hers). The tests all look at cell-free fetal DNA - pieces of baby’s DNA that pass through the placenta and circulate in the mother’s blood. Such disorders include trisomies, conditions where a baby carries an extra copy of a chromosome and which can be fatal or severely debilitating. The privately manufactured tests, which include Sequenom Inc.’s MaterniT21 and Illumina’s verifi, flag babies likely to have major chromosomal defects. ![]() “We tried to figure out if there were some that could be fixed,” Gammill said. Nobody knew how those false positives were arising, but the researchers had a few ideas. ![]() “It was hard for people to accept that things were really normal even when they were.” Built on a faulty assumptionīecause of the emotional toll of false positive results, Shendure, Gammill and their colleagues decided to take a closer look. “Because the test performance accuracy was billed as so high, when there was a false positive, the impact was durable through the whole pregnancy,” Gammill said. Gammill said her OB/Gyn colleagues saw this first-hand with women visiting the UW obstetrics clinic for prenatal care who had diagnostic tests done that definitely verified the existence of a problem, or confirmed the fetus was normal. Those false positives are a big issue, Gammill said, because of the way the tests are marketed. Hilary Gammill, a Fred Hutch and UW obstetrics researcher who ran the study along with UW geneticist Dr. Importantly, the test manufacturers could immediately alter their analysis to rule out this particular type of false positive, said Dr. Viruses, Vaccines and Infectious Diseases.Institutional Partners & Collaborations.Vaccine and Infectious Disease Division.
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