Simple Test Could Detect Down's Syndrome

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Nov. 21, 2000 -- Doctors have shown they can detect Down's syndrome at an early stage in pregnancy simply by taking a tiny sample of the mother's blood.

The test could give parents an earlier indication of whether their baby has particular chromosomal abnormalities and eventually may do away with existing methods of detecting those abnormalities in the developing baby in the womb -- some of which carry a risk of miscarriage.

It involves looking at the mother's blood to try and find genetic material from the baby that can then be studied for abnormalities using existing laboratory testing techniques.

Researchers from Hong Kong who used the testing method say they hope it will lead to safer and more reliable ways of diagnosing fetal abnormalities.

Reporting their findings in The Lancet this week, they write: "Ultimately, prenatal diagnosis by this technique could reduce our reliance on invasive methods."

The vast majority of Down's syndrome cases are due to an accident of nature, a chromosomal abnormality that can happen to anyone, while a fewer number of cases are due to a genetic problem inherited from either the mother or father.

The most common way of diagnosing it at the moment is amniocentesis -- a test that involves inserting a fine needle through the abdominal wall to the uterus to remove a small quantity of the amniotic fluid. This fluid then undergoes biochemical and chromosomal analysis to spot certain abnormalities in the fetus.

The procedure normally is carried out between the 15th and 20th week of the pregnancy. It is often distressing for the mother because chromosome tests can take several weeks to process. This means that, if the mother opts for a termination because the results show there are defects, it has to be performed when she is well into her pregnancy.

Because of the risk of miscarriage, the test normally is only offered to women aged over 35, when the risk of Down's syndrome exceeds the risk of losing the baby because of the amniocentesis, or if there is a family history of abnormalities.

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Some women are offered an alternative test -- known as chorionic villus sampling (CVS) -- which can be performed much earlier than an amniocentesis, from around 12 weeks, where a tiny quantity of tissue can be taken from the placenta. However, this too carries a risk of miscarriage.

The hope is the new technique will be easier and safer. Scientists at the Chinese University of Hong Kong identified three women who were carrying fetuses already known to have the most common form of Down's syndrome, and 10 others who were carrying babies with normal chromosomes.

At 12 weeks gestation, doctors took a tiny sample of each mother's blood. Then they took another sample at 15 weeks, after some of the women had undergone CVS testing. The reason for this was that CVS could introduce fetal cells into the mother's blood during the procedure. The scientists wanted to see if the same cells were already there before the pregnant women underwent any tests.

Using a technique called fluorescence in-situ hybridization (FISH) -- where fluorescent markers are used to paint a picture of the part of the chromosome under investigation -- the researchers were able to pinpoint fragments of fetal DNA.

In all three Down's syndrome cases, they were able to detect the chromosomal abnormalities from the mother's blood early on in the pregnancy. After each woman had CVS testing, they checked the samples again and found the same defects.

"These results indicate that these cells were present in maternal plasma before the invasive procedure," the researchers write. "We have shown that prenatal detection can be accomplished by FISH analysis of fetal cells harvested from maternal plasma. The procedure is simple, and our data suggest future large-scale trials should be initiated to assess the diagnostic accuracy of this method."

Despite the fears of miscarriage, tests like amniocentesis and CVS are relatively safe, says Deborah Driscoll, MD, associate professor of medicine at the University of Pennsylvania's department of obstetrics and gynecology in Philadelphia. But because of the miscarriage fears, doctors are looking to make more options available. That is what's motivating investigators like the ones here to look for tests that are easier to do and come with lower risks than those commonly used today.

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As for this research, "this is very preliminary and interesting, but it's still too premature if this will be clinically useful," says Driscoll, a geneticist and obstetrician at Penn.

"Clearly it is easier to do this test, but whether it will be as sensitive [as the standard tests today] remains to be seen," she tells WebMD.

Driscoll says that CVS and amniocentesis give more information than just the signals of Down's syndrome. Doctors get the number of chromosomes present and they can see the chromosomes' structure. Clinicians are not going to stop doing these standard tests and lose all that other important information in order to do a test for just one genetic abnormality. But the study should spark more research into finding more testing options for future patients, she says.

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