HOW DOES IT WORK?
A few years ago, it was discovered that “free-floating, non-cell-bound DNA from the foetus is present in the mother’s bloodstream,” explains Dr Jaysen Knezovich, laboratory director and medical scientist at Genesis Genetics South Africa. This discovery offered scientists the intriguing possibility of studying the mother’s blood (but actually the foetus’ DNA) to discover facts about the baby she was carrying.
Our DNA is packaged into structures called chromosomes. We normally have two copies of every chromosome, with 46 chromosomes (in 23 pairs) in total. Down syndrome, for example, is characterised by the presence of one extra chromosome. “Imagine 46 volumes of an encyclopaedia on a bookshelf,” says Dr Knezovich. “It is relatively easy to count them and see if there is one extra volume.” This is what NIPT does – it counts the chromosomes in the foetal DNA sourced from its mother’s blood. NIPT is over 99 percent accurate in picking up Down syndrome and it is indeed most often used for this purpose.
Other genetic conditions, which might be buried deep inside a few gene mutations, are harder to spot than chromosomal ones. “To screen for every condition and genetic abnormality we know of would be like having to proofread the entire 46 volumes of the encyclopaedia, and being expected to spot a spelling error on page 4678 of volume 21,” says Dr Knezovich. “Unless you know what you are looking for, it is virtually an impossible task.” Because of this, NIPT is only used to count the number of copies of a few chromosomes, which are associated with genetic conditions such as Down syndrome.
However, the sex of a baby can also be read from its chromosomes – pairs of XX chromosomes are female, XY pairs are male. This is why NIPT is sometimes also used for knowing if you are carrying a girl or a boy – and its accuracy is 97 to 99 percent. A study conducted last year by the UK’s National Institute for Health Research (NIHR) found NIPT to be safe and effective, and this has opened the test up to broader usage internationally. Whereas you could obtain the test three years ago in selected centres only, it is now available across South Africa, and the cost, while still in the region of a belt-tightening R7 000, has come down from the previous R15 000 – and can be expected to fall further in future, like most new technologies do.
WHEN IS IT DONE?
NIPT is done from 10 weeks of pregnancy, which is earlier than the second trimester testing of amniocentesis or CVS, and leaves more time for parents to decide whether they want to continue a pregnancy, or what treatment options, even prenatal ones, and birth options might be available for them. But does NIPT completely replace amniocentesis and CVS? No. As Dr Knezovich says, “amniocentesis and CVS remain the gold standard diagnostic tools presently.” This means only these tests can be said to diagnose the presence of a condition positively. “But when a patient comes in and her ultrasound and blood test results show suspicions of an abnormal pregnancy, and that there is a good indication for further testing, then NIPT is an excellent screening tool as the next step. A positive result for a chromosomal abnormality on NIPT would then definitely lead to a recommendation for an amnio or CVS to confirm the result, and a negative NIPT result would generally mean no further tests are needed,” says Dr Knezovich. YP