NIPT tests v NHS tests

[Sophie4113]

Can anyone help me work out whether it is worth having a NIPT test or just having the NHS blood tests for chromosome abnormalities? What is the difference?

As far as I can work out, both screen for Downs, Edwards and Patau, both involve a blood test and neither gives you a definitive yes or no answer, just a high or low risk score. So it seems that the only advantage of having a NIPT test is that you can have it at ten weeks, rather than at about sixteen weeks on the NHS.

Am I missing something?

I am nervous about paying for the NIPT test at ten weeks when there is still a significant miscarriage risk. Do they scan you first to check for a heartbeat?

Thanks x

Hi @Sophie4113

The difference between them is the accuracy of the test. The combined one includes a component of maternal age, so if you're in your 30s you'll automatically get an elevated risk factor (and the older the worse it is - as is the false negative risk). The NIPT tests look for foetal DNA so actually look at the baby's genetics rather than trying to work out what's going on from hormone levels in your blood. You can also choose to find out the sex if you want to.

Www.arc-org.uk has some really useful information on it.

It really does depend on your own attitude to risk though. For us, we wanted to know ASAP so that we could choose what to do next, and I'm also 39 so would have had an elevated risk due to that. I had mine at 11 weeks and yes, they do a scan first to check the pregnancy is viable before they do the NIPT.

Good luck OP!

The nipt test has a 99% accuracy rate.
The nhs combined test has an 85% accuracy rate

Thanks both! X

As above, the NIPT is much more accurate. If you were considering a termination based on the results you would be offered the chance for a confirmatory cvs or amnio.

As above- the NIPT is based on actual foetal DNA, whilst the combined test is just based on hormone levels that, if a bit unusual, can be a sign of a chromosomal abnormality but can also just be normal for you/your pregnancy.

The accuracy rate of 85% vs 99% is (roughly) how many babies with a chromosomal abnormality the combined and NIPT screenings pick up respectively, but what I think is another important statistic for your decision making is the false positive rate.

Something like 5% of all women who have the combined screening receive a “false positive”, whilst it is very rare with the NIPT. Like many women who have posted here, I have had a “false positive” high risk combined screening result which resulted in a horrific week having the NIPT and awaiting the results, and I resolved that if I were to be pregnant again I would go straight for the NIPT.

However, I have read somewhere that a benefit of the combined test is that it can be helpful in identifying out of range hormones (bHCG and PAPP-A) which can also identify issues with the placenta/growth.

@MajorFaffington I've also seen the comment about the hormones being useful for other issues, but I'm curious as to why if they are bothered by this, they don't suggest that those bloods anyway for those who opt out of the Downs screening 🤔

Thanks everyone. I will have a think about it x

@Weathergirl1 You’re right, I have never heard of anyone being advised to still have combined test after NIPT for that reason either.

All the best with your decision and the rest of your pregnancy @Sophie4113

Thanks. I thought of another question. Does anyone know why the NIPT tests can't tell you yes or no for these conditions with 100% accuracy, rather than just giving a risk factor? If they are looking at foetal DNA, you would have thought they would be able to say a definite yes or no.

No it isn't 100% but it is 99.9%. The only way to be absolutely certain is CVS or amino.

@Sophie4113 I believe it's because the diagnostic tests (CVS and amniocentesis) sample the DNA directly from the placenta/ amniotic fluid )so effectively the baby) whereas the NIPT tests collect bits of foetal DNA that's floating around in your bloodstream to test, so there's still a small chance they might not see the bit they're looking for. For example, when we had ours they said that if the couldn't detect X or Y then they wouldn't retest if the trisomy chromosomes had been detected fine (as sex determination is a bonus) and that boys are easier to detect with certainty as the only way a woman would have Y chromosomes in her blood is if it was from the foetus!

What many people do not understand and what was delivered to us as part of NHS midwifery NIPT training is that NIPT is only 99% accurate on people who have already been declared high risk by combined or quadruple screening.

The other thing is that if you plan to terminate a pregnancy with a positive result, it would be deemed a "social abortion" unless the congenital abnormality was confirmed by invasive testing. So, for me, if I knew I would likely terminate, I'd skip NIPT when it comes out on the NHS and head straight for amniocentesis or CVS if I had a high chance of a trisomy. Otherwise I'd use NIPT to avoid the risk of miscarriage but still have a very good idea about whether my baby will have any of those conditions.

This also means that I'd only seek NIPT privately if I had a high risk combined screening AND I was not likely to terminate. If I was likely to end the pregnancy, I'd have NHS amnio

Private companies do not tell women that NIPT is about as effective as combined screening on the general population which means some affected babies will be missed. I've looked after 2 women who had this experience. Both were devastated and shocked that NIPT isnt 99% accurate on the general population.

@Antecedent that's interesting I hadn't heard that. So if you've have a low risk on the combined test and then had the nipt (because of a ? on a scan but didn't want an amnio because you didn't want to MC risk) came back high risk what would be your thoughts?

It would depend on whether I was likely to terminate. If I was not, I'd bear in mind that baby is likely to have a trisomy and continue with that path. If I was likely to terminate, I'd have the amnio. But that is only because I'd be irrationally bothered by the termination not being classed as "medical". Some people wouldn't care and would be willing to act on a high chance NIPT +/- scan or other signs all isn't perfect. I think my view on needing all involved to know it was for medical reasons is more about my own limitations than anything else. It shouldn't matter as long as I feel I've made an informed choice which is right for me and my family (including the baby in question).

That's all rather contradictory to what the NHS fetal medicine consultant told us - he agreed that NIPT is much better than the Combined Test especially for false negatives and for older mothers who will have skewed results due to the age component. Clearly if high risk comes out at all, then the only way to be 100% sure is for diagnostic testing.

I got the impression that the only reason the NHS still does the Combined Test is that it's cheaper than NIPT for the whole cohort of women going through the system.

NIPT is coming out in the NHS as an option for people who have high risk combined screening. My Trust have already trialled it and how it will work. I think what your consultant meant is that it gives a better idea if you're "high risk" on Combined screening (which is anything above 1/150) but when tested on every single pregnant person, it doesn't give much of a different picture than combined screening. It definitely gives a more definitive answer to people who are already high risk but not to the average pregnant person. We had to be able to explain that because a lot of our 40+ mothers wanted it instead of combined screening and we had to point out that they could miss what they want to know if they don't do both.

As I said, we had 2 women in quick succession. One left the baby with us.

NIPT is offered by some NHS trusts. It’s a postcode lottery. Also note that if the ultrasound part of the Combined Screening doesn’t work, you will need to have a Quadruple Test. That comprises of a blood test that looks for Downs (it’s about as accurate as combined screening: 81% vs 85%), and extra scans to manually search for Edwards / Patau and other trisomys / defects (usually as an extra long 20 week scan)

No, that is not what the consultant said at all. I thoroughly researched these tests before we took the plunge and looked at the scientific literature, and your statement is at odds with what I read, so I would be interested to see the source behind what you have said, unless you are lumping together older women as being automatically high risk? Emily Oster's book also clearly shows (from scientific studies) that the false positive and false negative rates of the combined test are higher with increased maternal age.

@Antecedent Are you able to explain why the NIPT is less accurate for women who aren’t already high risk?

My understanding about accuracy was the same as @Weathergirl1 has stated.

My mw told me the same as @Antecedent but I was later on in my pregnancy. My mw said NIPT sensitivity depends on the disorder & the gold standard if you are past wk 14 is still a consultant manually looking for deformities / problems via an ultrasound .

I think it comes from this paper which details the misleading quoting of statistics and mis-use by clinics
nuffieldbioethics.org/blog/nipt-private

You then also have this article from the BBC:
www.google.com/amp/s/www.bbc.co.uk/news/amp/stories-47150878

Wow, Nuffield quotes the PHE leaflet as providing balanced information... The version I was given doesn't mention the false positive/negative rates for the Combined Screening tests and judging by the worried posters you see on here, many don't understand the implications of those.

I think this article is why many reputable clinics refer to NIPT as a Downs test (and even then results are about as accurate on the general population as a Quad or Combined test). I was going to book it and was told that actually it was worse than the Combined for detecting Edwards / Patau and the other trisomies but that the gold standard is still manually searching for markers via ultrasound with amnio if required.

I think in terms of false positives it’s the same overall as the Combined. Remember - the Combined test’s published statistics are an average across all trisomies but it’s better at detecting everything except Downs than an NIPT.

The major NHS study done by GOSH that basically informed the rollout of NIPT looked at women who were at medium or high risk (1/150-1000) and concluded that it would only increase Downs detection in people who were high risk but would increase uptake in women who were at medium risk (and often choose not to have invasive testing thus are most likely to unexpectedly have a baby with Downs).

I found this particularly interesting because I think it is helpful to prepare beforehand (the studies show that women do use NIPT for more information and not purely to decide whether to terminate), but I also struggle with people who terminate based on a suspected Downs diagnosis which might not be accurate and/or have a false impression of Downs Syndrome and the quality of life and independence people can achieve. So I'm for anything that allows people to safely prepare and gather information whatever their final choice will be.

While you are busy judging people @antecedent, bear in mind that despite what some exceptional people with Downs Syndrome achieve, 50% get dementia, 15% get Leukaemia, many have severe mental health problems, many have severe autism, many have severe/fatal heart defects, many have severe bowel/digestive tract issues. 50% of babies with Downs die in utero (not counting terminations).

When you are faced with a real decision, rather than a theoretical ideal of what might be, it's a little harder to risk that life for your unborn child and other children.

I know many people with downs through my work and personal life. I always thought I could never abort for Downs. I didn't have the combined test but my 12 week scan was too abnormal for the sonographer not to act. Realising ending my pregnancy was the right thing (in our set of circumstances and experiences) was incredibly painful. I have never encountered anyone who took the decision without being informed, and heartbroken.