November 17, 2011

Prenatal blood test offers new Down syndrome screening tool

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Vanderbilt is offering a highly accurate prenatal blood test that can detect a chromosomal anomaly associated with Down syndrome. (photo by Jenny Mandeviile)

Prenatal blood test offers new Down syndrome screening tool

A non-invasive prenatal blood test that detects the chromosomal anomaly that is the most common cause of Down syndrome is now available as a screening tool for pregnant patients at Vanderbilt University Medical Center.

The test, marketed by the company Sequenom Inc., is called MaterniT21 and has demonstrated high accuracy (99.1 percent) detecting positive cases of fetal Trisomy 21. It’s available in only 20 major metropolitan regions across the United States.

The blood test is intended for women at high risk for carrying a fetus with Down syndrome (about 750,000 a year) and can accurately test maternal blood as early as 10 weeks gestation. There are about 400,000 people living with Down syndrome in the United States, which occurs in one of every 800 deliveries.

High maternal risk includes women with “advanced maternal age” of 35 and older, those with a fetal ultrasound abnormality, a positive first or second trimester screening or a family history of Down syndrome. Most PPO plans will cover all but $235 if a woman has any of these risk factors.

Medicaid and most HMOs require pre-authorization. If no risk factors are present, the blood test costs about $2,000 out of pocket.

At this point, it doesn’t replace amniocentesis or chorionic villus sampling (CVS). Patients with a positive result from the blood test should also have amniocentesis or CVS for confirmation.

Amniocentesis, done after 15 weeks gestation, has a one in 300 miscarriage rate and CVS, done at 10-13 weeks has a one in 100 risk.

“The blood draw is a screening test at this point, but in three to five years it will be a diagnostic test,” said Frank Boehm, M.D., professor of Obstetrics and Gynecology and vice-chair of the department.

“What patients need to ask themselves before they have this test is, do they want this information?” said Martha Dudek, a genetic counselor in Obstetrics and Gynecology. “There’s a concern that in many patient settings, patients won’t really be informed once they elect to have the testing done. There’s no turning back once you have the information.

“We feel strongly that those who are interested in this test should meet with a genetic counselor prior to this test,” Dudek said. “Typically patients would only see a genetic specialist prior to invasive testing.”

The blood test is not done at Vanderbilt, but patients are sent to a lab in Brentwood to obtain a small blood sample, which is mailed to the company’s San Diego lab for analysis. Results are available in about eight days.

Currently, the test is only for Trisomy 21, but it should soon cover Trisomy 13 and 18 as well, Boehm said.

Trisomy 13 and 18 are genetic disorders that include a combination of birth defects including severe intellectual disability, as well as health problems involving nearly every organ system in the body. About 90 percent of babies born with trisomy 18 or 13 die by age 1.

Boehm said there’s a concern by Down syndrome advocacy groups that with earlier diagnosis of Down syndrome there will be more abortions with these pregnancies.

“However, in Tennessee, we’re the buckle of the Bible Belt and the vast majority of people have the feeling of ‘whatever God gives me, I will be taking home,’” he said.

Dudek said only about 10 percent to 20 percent of Vanderbilt’s obstetric population chooses to have prenatal testing.

“What’s important from our aspect is ensuring that patients still feel autonomous in their decision making and that they don’t feel pressured to have testing, period.”