Non-Invasive Prenatal Testing

by Colette Lloyd

Advertising is a dangerous thing. It can be used to inform people but also to mislead. We do need to know about someone’s product, but sometimes, in order to sell a product, advertisers go too far.

This happened in relation to non-invasive prenatal testing, and finally, in December 2019, followed by an enforcement notice in early 2020, the Advertising Standards Authority have recognised it. https://www.asa.org.uk/resource/enforcement-notice-nipt.html

Dispelling the myths

Non-invasive prenatal testing uses fetal DNA, but fetal DNA does not translate to being the baby’s DNA. It is actually placental DNA that is tested. Why does this matter? Because the placental DNA can be different from the baby’s DNA, (although both are produced by fetal DNA), in a condition called placental mosaicism. This is one of the reasons that NIPT is unlikely to ever be 100% accurate. However, calling it the baby’s DNA makes NIPT seem more accurate than it is.

“It is actually placental DNA that is tested.”

Which brings me to accuracy. When a woman asks, “How accurate is the test”? what she is really asking is “How likely is it that my result will be correct?”. When a clinician asks it, they want to know “How many of all the babies with Down syndrome there are, will the test pick up?” Two very different questions. The answer to the second question is within the region of 97-99%. However the answer to the question women are actually asking is the Positive Predictive Value. And if you are in your 20s, the positive predictive value is as low as 46%. On a yes/no, 50:50 question. But those figures don’t sell tests. They might however, prevent a woman having unnecessary investigations during their pregnancy, or worse, an abortion based on misinformation. There is a calculator here for your PPV https://www.perinatalquality.org/Vendors/NSGC/NIPT/

“And if you are in your 20s, the positive predictive value is as low as 46%.”

Therefore, considering these “accuracy” figures, you definitely can’t avoid an amniocentesis by having NIPT, unless you don’t really want to know for sure. In which case, you may want to ask yourself why you are having testing in the first place, unless you like to gamble. The test can be wrong both ways. It can tell you your baby does have Down syndrome when she/he doesn’t, and it can tell you your baby doesn’t have Down syndrome when she/he does. Therefore you do still have to have an amniocentesis to know for sure.

The safety statement is a red herring. It is safer than an amnio, but it doesn’t give you the same results as an amnio, so it isn’t really comparable. It is as safe as current screening tests which are also screening tests, just like NIPT.

And lastly to finding out earlier. Amniocentesis is safest when done after 15 weeks. There is another diagnostic test that can be done earlier called CVS, however, remember that placental DNA? CVS also tests placental DNA, so if NIPT is incorrect, CVS might be too. So you still have to wait for an amniocentesis to be sure. Sadly, not everyone has understood that, and the advertisers should be ashamed (The Journal.ie, 2019).

“…the advertisers should be ashamed”

For a copy of the leaflet for you or your healthcare professional click on the image below: