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Down Syndrome screening results - how are they calculated?

(15 Posts)
zambean Sun 02-Oct-16 10:42:21

I received my results from the combined screening for the trisomies last week, and my risk for Down's was calculated as 1:23.

I went in to the hospital to talk to the consultant, but it wasn't especially helpful, and I don't feel I have any understanding of how they actually arrive at that figure.

I understand the various factors they take into account, and for the record, here are my results:

Age: 35
NT: 2.4mm
HCGb: 1.98 mom, 2.58 corrected mom
PAPP-A: 0.3 mom, 0.49 corrected mom

So I get from this that my NT is on the high end of normal, my HCGb is about two and a half times what it should be, and my PAPP-A is about half what it should be.

But what I would like to know is - how do they use all this information to arrive at a number like 1:23?

Does each of these things give me a risk factor of 1:x, and then these are all multiplied together to arrive at a combined risk factor?

The reason I ask is because literally nobody seems able to explain how they arrive at these figures, not even my consultant. I sort of get the impression that only a handful of people in the world might know what's going on, and that what happens is that they collect all this data, someone in an office somewhere puts it into a computer program that spits out a number, but none of the medical professionals we actually come into contact with really understand how that number is arrived at or how reliable it really is.

Any thoughts?

strongandlong Sun 02-Oct-16 10:58:24

Sorry you're dealing with this. it must be very stressful.

My understanding is that you're right. The result of each test are combined to produce an overall risk. the calculation will based on pretty complex statistical analysis, so I'm not surprised that the professionals involved can't explain the detail.

I suspect that the number of tests done = lots of base data = the calculations are very accurate these days.

But this is only a risk, not a diagnosis. Are you going to have a diagnostic test?

This is fairly detailed, but doesn't explain the calculation.


zambean Sun 02-Oct-16 11:05:48

Thank you for that link, it's really helpful.

I'm not going to have invasive testing because of the risks involved. I have had NIPT and am just waiting for the results.

This waiting is absolute torture, it's pure hell. I feel like I'm on death row.

zambean Mon 03-Oct-16 19:12:43

I found this slideshow quite interesting, posting here in case others do too

zambean Mon 03-Oct-16 19:43:38

And this, from Prof Nicolaides, which is incredibly helpful

In particular:

Sequential screening
• Every woman has a risk that her fetus/baby has a chromosomal
• The background or a priori risk depends on maternal age
and gestation.
• The individual patient-specific risk is calculated by multiplying
the a priori risk with a series of likelihood ratios, which
depend on the results of a series of screening tests carried out
during the course of the pregnancy.
• Every time a test is carried out the a priori risk is multiplied by
the likelihood ratio of the test to calculate a new risk, which
then becomes the a priori risk for the next test

(page 15)

strongandlong Mon 03-Oct-16 22:33:00

Hope you're bearing up ok, OP.

When are you expecting he results of the NIPT?

Thats a good, clear explanation from prof Nicolaides.

NeverAShadowOfDoubt Tue 04-Oct-16 12:08:07

Hope you're doing okay OP. I just wanted to share my experience with my current (only) pregnancy. I'm 35 years old and 21w 4d with boy girl twins, I didn't find out it was twins until the 12 week scan. That was a big shock in itself! But then came another big shock, twin B had an increased NT of 6.1 and there was mention of hydrops too. The hospital whacked a shedload of negative information on us, we were taken to 'that room' with the flowers and the box of tissues and they clearly were expecting the worst case scenario and we were spoken to about selective reduction of the pregnancy etc. I then had the bloods taken for the combined screening and the bloods came back giving twin B a risk of 1:14 for downs and 1:30 for edwards/pattau. We were devastated. Twin A on the other hand whose NT was 1.6 had a risk of 1:38000 for Downs and 1:14000 for edwards/pattau. I however did not find out the readings from the bloods so not aware of the HCGb and PAPP-A levels. We were referred to the fetal medicine unit within 3 days and from there the consultant rescanned me and remeasured the NT as 4.4 for Twin B. Still increased but not as high as the NT the regular 12 week scan showed. Consultant said there was no evidence of hydrops at all. Made us think that perhaps the 12 week sonographer wasn't accurate at all. Anyway, we decided to have the Iona test (non invasive) and after a week, our results came through as Low Risk for all 3 syndromes, in fact low risk as in less than 1:1,000,000. So I just wanted to share this with you to give you some hope. My 16 and 20 week scan showed that Twin B's NT was back to normal. It's a hellish tough road and we have been on a roller coaster but it just goes to show that quite often these things all amount to nothing. Stay positive love smile.

TheHubblesWindscreenWipers Tue 04-Oct-16 12:27:40

It's basically an equation- they plug in your age, weight (more on that in a mo) and those results in and out pops the probability. They won't be able to explain it because it's a black box to them - it's hardcore maths 🤓

They need the weight because the bloods can vary with weight. NOT because weight itself affects the result itself, if you see what I mean.

When I was pregnant I tried to find the algorithm (hello fellow nerd!) but I couldn't. It seems to be pretty complex - here's a link to a software that does it for example.

1:23 is still an almost 95% chance of normality.

Your next question is how reliable it is. That's complicated too. All screening tests have the possibility of false positives and false negatives. You need an actual diagnostic test for a yes/no. If we are being really pedantic, even amniocentesis doesn't guarantee that because of the risk of picking up placental mosaic cells for example. As you age, the test picks up a higher percentage of cases but also through up more false positives. For example at 30 the false positive rate is 4% but at 49 it's 67%. Link below

The new nipt testing is fantastic - it is strictly speaking a screening test but it's very very accurate. We had it. It tests not just for Down syndrome but also for the most common chromosome abnormalities and (some versions of it like panorama) test for other common genetic glitches like translocations.

Whether you wish to progress with the pregnancy or not is of course an intensely personal call and not one that anyone other than you and your partner can make. It is valuable to know one way or another though because it means that if you do progress the team looking after you can plan for any additional support needed at birth (cardiac complications etc.)

Really hope you get the results you want. flowers

Seriousjockin Tue 04-Oct-16 12:36:11

Sorry you're going through this stressful time OP, it really is a very anxious wait for the NIPT results. I'll be watching this thread though, because I am also really keen to know how they arrive at that figure. Nobody has been able to explain it to me and I have asked every single healthcare professional I have seen!

We were given a very high risk. (By the way, our NIPT came back all clear and our risk was much higher than yours, so you honestly have a very, very good chance that your baby is unaffected by any of these conditions. We found it helpful to think of it in percentages, a 1:23 risk is something like a 96% chance that everything is absolutely fine).

Good luck, hope your results come back soon.

TheHubblesWindscreenWipers Tue 04-Oct-16 12:48:14

No one will be able to explain it - it's a complex algorithm that needs a computer to run 🤓

It's basically a series of equations - you plug each result in and it does multiple regression analysis (I think)

Here's a paper. Enjoy smile

zambean Tue 04-Oct-16 15:02:59

I actually found that document that I linked to above, by Prof Nicolaides, very helpful and it explained a lot. It's long, but I really recommend reading it, if you are interested Seriousjockin.

This waiting is the absolute worst thing that has ever happened to me. Am trying to prepare for the worst, and prepare myself for the possibility of termination.

curliegirlie Tue 04-Oct-16 20:39:06

Zambian, do watch the Sally Phillips documentary on Down's Syndrome tomorrow on BBC2 at 9 pm.You might find it an eye opener! Also, have a look at the Down's Syndrome Association website for the information they have for expectant/new parents. It's excellent, rounded information, whereas I find doctors etc. tend to dwell on worst case scenarios....

If your child has Down's, it's NOT 'the worst'. Yes, it's certainly a different journey, and there can be a number of associated medical problems, but my 13 month old has no heart or bowel problems, her hearing seems ok so far, and she is keeping up pretty well with her NCT peers. She's just started crawling and is into everything! Finding out she had Down's was a huge shock, as we were given 1:1900 following the NT screening/bloods, but I can't imagine life without her.

Good luck, whatever your decision 🌼💐.

zambean Fri 07-Oct-16 10:19:33

Well, when I was waiting for my results I read through every single one of these threads, scouring them for positive stories to comfort myself with, so I guess I should pay that favour forward and share mine!

After getting this risk of 1 in 23, I immediately went to This Is My in central Manchester and had the Serenity test, a form of NIPT. Yesterday - a week later - my results came back all clear for the three trisomies, as well as for Klinefelter's and Turner's syndromes!

To say I am happy and relieved is an understatement. I'm elated. The major lesson I've learned is, not to bother with the NHS screening again. Next time, I will have NIPT at ten weeks, and just have the NHS ultrasound at 12 weeks without the screening.

I hope everyone gets good news!

NeverAShadowOfDoubt Fri 07-Oct-16 10:43:03

Brilliant news Zambeam smilesmile

TheHubblesWindscreenWipers Fri 07-Oct-16 11:12:01

Fantastic. Really pleased for you.

Just a note that at ten weeks it's possible the foetal DNA levels are a bit low - we had nipt (panorama) and there wasn't enough DNA at 11.5 weeks to do both the main trisomies and all the deletions etc. next time I'll wait until 12 weeks I think. Of course it could have been a sample taking error...

Anyway, very glad you had good results!

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