Thu 7 Mar 2013
A non-invasive maternal blood test that can detect certain fetal chromosomal disorders, including Down syndrome, early in pregnancy is gaining attention as a potential new method of prenatal screening.
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Currently screening for Down Syndrome and other disorders caused by having an abnormal number of chromosomes, is carried out by either a combination of blood tests and an ultrasound examination or by more invasive testing such as chorionic villus sampling (CVS) or amniocentesis. This current screening method for fetal chromosomal abnormalities measures the amount of certain markers in a maternal blood sample combined with results from an ultrasound and is usually performed between 10 and 11 weeks of pregnancy (see First and Second Trimester Down Syndrome Screen). It can correctly identify approximately 85% of women carrying a fetus with Down syndrome with a false positive rate of 5%. Chorionic villus sampling and amniocentesis are diagnostic tests rather than screening tests because they allow the number of chromosomes to be seen and counted so their diagnostic accuracy approaches 100% as long as the sampled cells can be grown successfully.
The new test is called Non-Invasive Prenatal Testing (NIPT) of cell-free fetal DNA (cffDNA) using Next-Generation sequencing. It requires only a blood sample taken from a pregnant woman's arm. The blood sample is tested for fetal cell-free genetic material that circulates in her blood. The tests are available from a number of providers, and screen for three genetic disorders: trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome) and some tests can also detect abnormalities of the sex chromosomes that cause disorders such as Turner and Klinefelter syndromes. The NIPT test can be done as early as the tenth to twelfth week of pregnancy, with results available in about 10 to 21 days, as the samples are sent overseas for testing.
The NIPT tests have different sensitivities and specificities for the different trisomies. Most of the available NIPT tests can identify 99% of cases of Down syndrome, with a false positive rate of less than 1% and slightly lower detection rates for trisomies 18 and 13 in pregnancies at high risk.
The American College of Obstetricians and Gynecologists (ACOG) released a position statement in December 2012 that called the new test highly accurate for screening but not for diagnosis. For now, ACOG says NIPT should be offered, after pre-test counselling, only to pregnant women at increased risk for trisomy 13, 18, or 21. They include: women age 35 and older, women with a history of a pregnancy with a child with a trisomy, and women carrying a fetus with ultrasound findings indicating increased risk.
ACOG says the test should not be offered to low-risk women or women carrying multiple fetuses because it has not been sufficiently tested in those groups. Additionally, ACOG recommends that women with positive NIPT results be referred for genetic counselling and be offered CVS or amniocentesis to confirm the diagnosis. The International Society for Prenatal Diagnosis and the National Society of Genetics Counsellors have released similar statements.
Associate Professor Stephen Robson, a vice-president of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) is reported in Medical Observer stating that; ”Like any newly introduced high-stakes modality you seek the testing from a practitioner… who can put the test in perspective for the woman [and] has a clear plan on how to manage an abnormal result.”
Laboratories offering these tests in Australia recommend the tests only for women at high risk of having an affected pregnancy due to factors such as advanced maternal age, previous affected pregnancy or high risk due to results of biochemical and/or ultrasound screening. Positive results on the NIPT tests will need to be confirmed by follow-up testing using chorionic villus sampling or amniocentesis.
These NIPT tests are not eligible for a Medicare rebate and the costs currently range from $1,250 upwards. At this early stage in the use of the tests in Australia, laboratories are expecting referrals to come from radiologists specialising in pregnancy scans and obstetricians since these specialists are well placed to provide appropriate pre- and post-test counselling to women undertaking the test.