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 The report was put together by a team of specialists in medical
genetics and published in Genetics in Medicine.
The ACMG made it clear in 2016 that labs should not offer this
screening if they could not follow the suggested guidelines, said
the new report's lead author, Dr. Brian Skotko, the Emma Campbell
Endowed Chair on Down Syndrome at Massachusetts General Hospital and
an associate professor at Harvard Medical School in Boston.
The tests, known as noninvasive prenatal screening, or NIPS, are
meant to be used as a sort of pre-screening so that women whose
fetus has a low risk of having a chromosomal abnormality won't need
to get more invasive tests, like amniocentesis.
The NIPS tests look at bits of DNA in the prospective mother's blood
that broke off from the placenta. The labs count up "free floating
fragments of chromosomes," Skotko explained. The theory is that if a
fetus has Down syndrome, for example, which is marked by three
rather than two copies of chromosome 21, many more copies of this
chromosome will be found in the mother's blood sample, as compared
to other chromosomes.
"When you find an overabundance of 21 then you're able to predict
whether or not it's because the fetus has Down syndrome," Skotko
said. But it's not an exact science and that's why "ACMG was clear
from the beginning that this was a screening test, not a diagnostic
one," he added.
Using the numbers from the blood sample, along with other factors
like the mother's age, the labs can calculate what percentage of
fetuses with similar results had Down syndrome.
The problem is that some labs are simply reporting results as
positive or negative. But there is a wide range of positive
percentages, and without knowing how high the number is, parents
can't make an informed choice, Skotko said. If for example, there's
a 95 percent chance that the baby has Down syndrome, then the
parents can either go on to have a diagnostic amniocentesis, which
carries a small, but real, risk of miscarriage, or they can decide
to educate themselves about the condition so they'll be ready when
the baby comes, Skotko said. "Expecting couples want to know the
percentage," he added.
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Another recommendation, seemingly ignored by some labs, was to
provide education to the prospective parents. This could include
links to websites that would provide information on the tests and on
the conditions the fetus might have.
"We want to shine a flashlight on their adherence or lack of
adherence so there will be a transparent way physicians and couples
can make the most accurate decisions," Skotko said.
The new report includes a table listing companies offering NIPS
plotted against the list of recommendations. Where a company has
followed a recommendation, the table shows a green dot, where they
haven't, a red dot.
"We want this big table to be replaced by one with all green dots,"
Skotko said.
The new report is "really interesting," said Dr. Leena Nathan, an
obstetrician-gynecologist and an assistant clinical professor at the
University of California, Los Angeles. "It was surprising to see
there was no standardization."
And that's very important, Nathan said, adding that the ob-gyns at
her hospital were about to have a meeting to discuss which lab would
be best to use. "This is a very helpful table," she said. "And we
will look closely at it and discuss it as a group as we decide which
company to go with."
SOURCE: https://go.nature.com/2I0yoJp Genetics in Medicine, online
April 3, 2019.
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