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 The new study, published in the New England Journal of Medicine, is
the largest to show the tests are accurate in even low-risk women.
But experts warned that women who test positive still need to
confirm the result through more invasive diagnostic testing such as
amniocentesis, especially if they would consider terminating a
pregnancy.
"This is a great test for detecting Down syndrome but it doesn't
detect everything, it isn't diagnostic, and it doesn’t always work
to provide a result," said Dr. Mary Norton of the University of
California, San Francisco.
Prior studies have shown such fetal DNA tests, which measure DNA
fragments from the placenta circulating in the mother's blood, are
highly accurate at detecting Down syndrome and two other chromosomal
abnormalities in high-risk women, typically those over the age of
35.
Several physicians' organizations have supported use of the newer
cell free fetal DNA tests over the standard screening in older,
high-risk women.
Norton and colleagues tested nearly 16,000 women who had an average
age of 30. The researchers compared Roche's Harmony test to standard
prenatal screening for Down syndrome - which relies on biomarkers in
the blood and a fetal ultrasound - in the same group of women.
The Roche test identified all 38 cases of Down syndrome compared
with 30 detected by standard screening. The false positive rate for
the new fetal DNA test was 0.06 percent of the study population
versus 5.4 percent for standard screening.
But there were still nine false positive results in the group that
got cell free fetal DNA screening.
In addition, in nearly 500 women the fetal DNA test was not able to
deliver any result because there was not enough fetal DNA in the
pregnant women's blood. Further testing, however, showed that some
2.7 percent of fetuses had chromosomal defects, including those that
could not have been detected by the new fetal DNA technique.
More than 1 million fetal DNA tests have been performed since 2011.
One of their advantages is that if women test negative, they can
avoid having an invasive diagnostic test, which can cause
miscarriages in roughly 1 in 600 women.
But the newer tests are not regulated by the U.S. Food and Drug
Administration, and companies are heavily promoting their
performance in ways that may mislead patients, critics say.
In a letter published in the New England Journal of Medicine, Ankita
Patel and colleagues from Baylor College of Medicine and the Chinese
University of Hong Kong warn that women should not decide to
terminate pregnancies based on such screenings alone.
They cited their own work that collected data on 307 women who
screened positive on fetal DNA tests. The results included 56 false
positives, meaning the child did not have Down syndrome or the two
other common abnormalities (trisomy 13 and 18) captured by the
tests.
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In some cases, women had already scheduled an abortion pending
amniocentesis results, a test given between 15 and 20 weeks of
pregnancy. Even when that diagnostic proved the child did not have
those abnormalities, some women questioned whether the screening
result was more reliable.
"They think it is a diagnostic test because it's genetic," Patel
said of the fetal DNA tests.
CONFUSION AMONG DOCTORS
Another reason for the confusion is the rapid introduction of this
new technology. Many doctors are still just learning about it, said
Dr. Michael Greene, chief of obstetrics, Massachusetts General
Hospital in Boston, who was not involved in the research.
He also points to companies' aggressive marketing of their tests on
the Internet. Ariosa Diagnostics, acquired by Roche in January, says
its Harmony test for fetal chromosomal abnormalities offers "clear
answers to questions that matter," promising "accurate results from
as early as 10 weeks of pregnancy."
Rival Sequenom Inc says on its website that it offers unambiguous
results: "There's no room for maybe." Other companies offering such
tests include Illumina and privately-held Natera.
Greene said many women are not aware that these tests are not
diagnostic and company disclaimers are not easy to find.
Both Sequenom and Ariosa said they recommend healthcare providers
counsel their patients that the screening test is not a replacement
for a diagnostic test.
The FDA is weighing whether to regulate such tests. Patel believes
the oversight might help because the tests "would have more
disclaimers so women understand what they are getting into."
(Reporting by Julie Steenhuysen; Editing by Michele Gershberg and
Diane Craft)
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