Safe Blood Test for Down Syndrome

Down syndrome testing is about to get easier and safer, thanks to a new prenatal blood test. Pregnant women can be screened as early as 12 weeks, and will have results in just a few days. The screening uses "massively parallel maternal plasma DNA sequencing to screen for fetal trisomy 21," says research published in the British Medical Journal this month. Only women who screen positive would be referred for more invasive diagnostic tests, such as amniocentesis or chorionic villus sampling (CVS) which carry up to a one percent risk of miscarriage.

Screening vs Testing: Will Blood Tests Eliminate Amniocentesis?

It is important for women to understand that screening is not the same as diagnostic testing. Prenatal screening tests are used to indicate women who are at elevated risk of having babies born with genetic mutations like Down syndrome, as well as other conditions like open neural tube defects. A positive screen does not mean the fetus is affected, but only that there is a significant risk. More invasive diagnostic tests like amniocentesis can then be used to confirm or rule out conditions like Down syndrome, cystic fibrosis, Tay-Sachs disease and sickle cell disease.

The new blood test for Down syndrome has a high degree of accuracy and produced no false negatives. The study authors suggest that this could mean some 98 percent of amniocentesis testing would no longer be needed, as only women who screened positive would be referred for more invasive diagnostic procedures. This projection could be a little too ambitious, however. "Down syndrome represents about 50 per cent of what we find," said obstetrician Dr. Mark Evans of Mount Sinai School of Medicine, "and this new test can help with that. But it may also give women a false sense of reassurance."

Universal Down Syndrome Screening Projected

Study authors indicate that Down syndrome blood tests could be available in the next two to three years, and project the tests would eventually be offered universally to all pregnant women. Because the risk of Down syndrome increases with maternal age, in some regions only women over age 35 or 37 may be offered tests such as amniocentesis today.

It may be longer before blood screening tests for Down syndrome are widely used, however, due to high costs compared to current technology. Tests in development could cost anywhere from $300 to $700, compared to an average of $100 for currently available early screening.

Universal Screening Could Reduce Maternal Options

Testing for Down syndrome has long been associated with pressure on pregnant women to abort a fetus that tests positive for trisomy 21, and if screening were to become universal in the first trimester there would likely be an increase in the number of women subjected to such pressures. A determination of Down syndrome leads to abortion in up to 90 percent of cases in the US, and the Canadian Down Syndrome Society reports its members often feel they were pressured to terminate their pregnancies.

Research comparing doctor and midwife attitudes towards noninvasive prenatal diagnosis (NIPD) with invasive prenatal diagnosis (IPD) showed that practitioners are less likely to ask for pregnant women to sign consent forms for noninvasive procedures, and are more likely to present information about these procedures on the same day the screening test is performed. With invasive tests, health providers are more likely to give the patient time to weigh the benefits and risks, and to consider whether she wants to have the test. This raises concerns that as we move towards more noninvasive procedures such as the new Down syndrome blood test, informed consent may take a back seat to universal screening. It is perhaps more important than ever, that pregnant women educate themselves about all the options available and that couples choose a health provider who is willing to carry on an open dialogue throughout the pregnancy.

Sources:

"Blood test for Down syndrome developed." CBC News

Rossa W K Chiu et al, "Non-invasive prenatal assessment of trisomy 21 by multiplexed maternal plasma DNA sequencing: large scale validity study." British Medical Journal

"Down syndrome." UCSF Medical Center

"Down syndrome blood test could cut invasive testing." CTV News

"Down syndrome: Trisomy 21." American Pregnancy Association

Ananda van den Heuvel, "Will the introduction of non-invasive prenatal diagnostic testing erode informed choices? An experimental study of health care professionals." Patient and Education Counseling