Quad test as opposed to triple test

[cornwallsharon]

So I went for my '12 week' scan last week, only to find out that I was 16 weeks gone - oops!

So they had explained all about the Down's testing and I took it all in, but when they realised how far gone I was, they couldnt do the nuchal fold measurement at the scan and I've had to have a 'quad' test instead of the normal triple test.

By that point I was a bit overwhelmed and didnt really take in what they were telling me.

So what is the difference between the triple and quad tests? They said I would get a phonecall within 3 days if the triple test had a high possibility, but that with the quad test it would be a letter in 2 weeks regardless of the result.

Is the quad test more or less accurate??

Thanks!

Quad is significantly less accurate. I think from memory it picks up 75-80% of downs cases, depending on how old the mother is.

Similar thing to me, I was 14+4 at my 12 week. No phone call as yet for a high risk. I think she said the latest would be a week from the scan, but that's probably area dependent.

Thanks for the replies! We have now received the results back and they were a 1 in 12771 chance of of a downs baby. What I don't understand is the relationship between the accuracy of the test and the probability result. So there is only an 80% chance that the probability is 1 in 12,000?

I'm still confused! (Although obviously reassured to some degree by the low risk factor we've been given).

That's actually a very incisive question :)

The 80% figure refers to the sensitivity of the test. That measures the proportion of true positives that are identified. So the quad test picks up about 80% of positive cases.

The specificity of the test identifies the true negatives - I.e. All babies without the issue.

Ideally you'd have a 100% specific and sensitive test but this doesn't happen.

They use a cut off off about 1:150 as a 'positive' trigger to offer amnio, so you have a low 'negative' result. (With the caveat that you can never say positive or negative for sure on a screening test!)

But what you want to know is what are the chances I'm in the missed positive group and the baby is affected? ?

From this webpage, which explains it very nicely: www.wolfson.qmul.ac.uk/service-1/antenatal-screening/screening-tests/the-quadruple-test

Detection rate :
80% of women with pregnancies affected with Down's syndrome will receive a screen-positive result. (The remaining 20% of women with pregnancies affected with Down's syndrome will receive a screen-negative result.)

False positive rate:
3.5% of women whose pregnancies are not affected with Down's syndrome will receive a screen-positive result. (96.5% of women whose pregnancies are not affected with Down's syndrome will receive a screen-negative result.)

Odds of being affected given a positive result (OAPR)
1:15 ?? Among women in the screen-positive group, 1 woman will have a pregnancy with Down's syndrome for every 15 who do not.