Fri 17 Apr 2015
Currently most pregnancy screening for Down syndrome and related disorders such as trisomy 13 and trisomy 18 is done by either first trimester or second trimester screening in Australia. However in the last couple of years non-invasive prenatal testing (NIPT) using a sample of the mother’s blood has become available in Australia although the actual testing is currently done overseas.
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This new type of testing uses tiny amounts of DNA derived from the baby that can be found in the mother’s blood. This type of testing has both advantages and disadvantages. The disadvantages are firstly cost, which is still expensive even though it has fallen from around $2,000 to $500 quite quickly. The second disadvantage occurs if this test is used instead of first trimester screening which includes ultrasound. This is because this ultrasound examination can detect a number of other abnormalities which are not detected by the NIPT.
A positive NIPT test result still needs to be followed up with an invasive diagnostic test such as chorionic villus sampling or amniocentesis because although rare, false positive NIPT results can occur. This is discussed in this NHS (UK) document.
There is potential for NIPT to expand to include a variety of other less common disorders that can be detected using circulating DNA from the fetus but this may introduce new problems such as a higher rate of false-positive results. Even now but very rarely, unusual results found in the test may lead to unexpected diagnoses in the mother instead of in the baby. In one case in the US, a mother who was also a doctor had unusual findings in the NIPT test. The laboratory contacted her obstetrician and suggested the possibility that the mother could have a cancer of some type but they could not identify what type or where it might be. The mother had a whole body MRI scan and this found a previously unsuspected colon cancer which was successfully treated.
In another case, doctors in Belgium identified abnormalities in a NIPT on a mother who was subsequently diagnosed with Hodgkin’s lymphoma. It is still early days for NIPT. It is not clear what place it should have in prenatal testing. Despite this the take-up rate in Australia has been rapid and may increase further when the Victorian Clinical Genetics service starts offering the test in the near future.
A discussion of some of these issues can be found in this position statement jointly issued by the American Society of Human Genetics and the European Society of Human Genetics.