There are now several prenatal blood tests available to the public that try to ease anxiety, at least when it comes to the health of the newborn. By analyzing tiny amounts of fetal DNA in the pregnant woman's blood, these tests are able to detect some genetic defects up to six months before birth. The decision whether to use these laboratory tests is of course up to the parents, but the results of the scan can be misleading
The article was published with the approval of Scientific American Israel and the Ort Israel network
The period of pregnancy often evokes mixed emotions: wonder and astonishment, but also concern. Will the boy or girl be healthy? Are you happy? Will he, or she, find their place in the world? There are now several prenatal blood tests available to the public that try to ease anxiety, at least when it comes to the health of the newborn. By analyzing tiny amounts of fetal DNA in the blood of the pregnant woman, these tests (marketed in the US [and also in Israel] under names such as Harmony, MaterniT21 PLUS or verify) are able to detect some genetic defects up to six months before birth. The decision whether to use these lab tests is of course up to the parents, but the scan results can be misleading, and the industry and federal regulators [in the US] are not doing enough to ensure that subjects get all the information they need.
Today, the tests identify major defects, such as three copies (trisomy) of the 21st, 18th or 13th chromosome, which causeDown Syndrome, for the syndrome Edwards and to the syndrome patao, respectively. These measures are undoubtedly an improvement in the safety of the test compared to previous procedures for testing the newborn's genes. In the past, it was only possible to detect such chromosomal defects through invasive tests, such as amniocentesis, which can cause miscarriage.
These genetic scans were originally only offered to women age 35 and older, who are at higher risk of having babies with Down syndrome or other genetic diseases. But today commercial companies also market these tests to low-risk pregnant women. In November 2015, a market research company found that sales of these tests in the US reached $2014 million in 563.4, an amount that is expected to quadruple by 4.
However, the federal regulations in the US do not keep pace with technological development. According to the laws in force in the USA today, such tests are considered laboratory tests, which, unlike new drugs, do not have to show a significant clinical benefit. All the manufacturers are required to do is demonstrate that their tests yield results within a statistically acceptable margin of error.
This standard made sense in the days when blood tests were designed to check for the presence of single compounds, such as sugar or certain hormones. Genetic diagnosis is different: it requires a much greater degree of interpretation and analysis to determine whether a suspicious result indicates a true genetic defect or just a harmless genetic variant.
First of all, the amount of fetal DNA found in the blood of the pregnant woman is tiny, so this DNA must be duplicated many times to get the amount of material needed for testing. This "amplification" process may, among other things, lead to an incorrect double count of chromosomes, i.e. create a false impression of the presence of an extra chromosome 21, for example. Another source of uncertainty comes from the fact that these tests are actually scans, which by definition are meant to deploy a wide network of tests. By its very nature, such a network yields more false positives compared to dedicated diagnostic procedures that are generally more accurate.
Ignoring this difference can cause serious problems. If a scan conducted on 1,000 pregnant women correctly identifies 19 out of 20 true cases of genetic defects, it is defined by statisticians as having a sensitivity rate of 95%. That sounds pretty good. But at the same time, the test may also yield 10 false positive results, meaning another 10 women will receive a result that will mistakenly imply a problem that is not there. The sensitivity rate will remain 95% because the test detected almost all the real defects. But only a little over 65% of all positives, 19 out of 29, will actually be true. This ratio, called in statistical language positive predictive value (PPV), is the measure of the degree of confidence we should give to the result of a particular test. However, most genetic screening companies do not provide the positive predictive values of their products. Instead they flaunt their sensitivity rates, which may mislead the subjects and even their doctors. These predictive problems are why anyone who gets a positive result from such a scan should have additional, more accurate tests.
The US Food and Drug Administration (FDA) must also follow these tests. He should speed up the processes to change the laws to oblige the manufacturers of protective scans to give more clinically relevant information, such as predictive values. The companies offering non-invasive prenatal scans must do more to educate us all about the possible downsides of the tests. And the parents-to-be must carefully consider whether they even want to have these scans, especially since new blood tests, which are supposed to give a glimpse of the fetus's complete genome, including an innate tendency to heart disease, cancer or diabetes, are just around the corner.
