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nuchal accuracy

(16 Posts)
BumblBeee Tue 16-Sep-08 18:59:27

Just had a scan and got a 1:1300 result. I was happy about this until I realised that the nuchal scan is only 95% accurate. This means there is a one in 20 chance that it was wrong?

I am finding this confusing. Surely this test is kind of pointless if one in 20 people get wrong results?

Am I missing something obvious here or interpreting this incorrectly?

thanks xxx

LIZS Tue 16-Sep-08 20:05:22

But it is only a statistical chance anyway, not an absolute value. Mosts stats will have a % margin of error. In this case it is partly due to possible inaccuracies in dates/measurements. However as long as your calcualted risk is lower than that purely based on your age I think you should relax, assuming you have no other reasons for concern in which case you could consider further tests.

BumblBeee Tue 16-Sep-08 20:34:50

I guess I am weighing a 1:20 chance the scan was wrong and the baby has downs (?) vs. a 1:100 chance of miscarriage with CVS.

It is a shame that I have not been able to see how I am meant to interpret the error rate anywhere.

lulumama Tue 16-Sep-08 20:38:44

that is a really low risk it is 1299 more likely your baby will not have downs syndrome

all tests have a margin of error, and low risk is not the same as no risk, only an amnio can give a definitive answer, but it is invasive, and your odds are really low

5% of women will get an inaccurate result, 95 % won;t

BumblBeee Tue 16-Sep-08 20:50:44

"5% of women will get an inaccurate result, 95 % won't "

yes 5/100 is the same as 1/20 which IMHO is quite high

lulumama Tue 16-Sep-08 20:55:55

if it was the reverse, i would be more concerned. there has to be a margin of error

was this not explained fully to you before you had the tests? there is an issue if you had the tests without fully udnerstanding the implications and the possible error for margin

is the 95 % figure country wide or is it for your particular hospital

there is a good website , ARC, i thikn, that can help with antenatal testing

you could get a 1 in a million result back, but you could be that one in a million. as i said, low risk is not the same as no risk.

can you call your midwife if you are really concerned? will you be having your triples test?

BumblBeee Tue 16-Sep-08 21:04:24

I will do a search for ACS.

My thoughts are that even if you have a 1:1000000 risk assessment it doesn't mean all that much if there is a 1:20 chance it is completely wrong.

BumblBeee Tue 16-Sep-08 21:12:21

ARC has some information but it does not tell how to factor the risks together.

I wonder if the 1:20 are mainly false positive or false negative?

BumblBeee Tue 16-Sep-08 21:14:26

FYI I had the test done privately today.

I had a scan last week at Homerton and they forgot to do the right blood test. The midwife also didn't know anything about CVS.

Encouraging NOT.

lulumama Tue 16-Sep-08 21:18:56

i don;t have the answers i'm afraid

can you call the clinic tomorrow and talk through your concerns...?

sorry you are so anxious

BumblBeee Tue 16-Sep-08 21:26:05

Yes I think I will call them. I am booked in for a CVS tomorrow morning but was hoping to make the decision today.

They said someone would phone my husband today but no one did.

I found it strange that no one could tell me how to interpret the statistics. I was at the FMC as well!

TotalChaos Tue 16-Sep-08 21:34:20

surely the 5% inaccuracy means that the rate might be different for the 1:1300 you have been given - so that it might be 1:450 or 1:2000 NOT that there is a 5% chance the baby has DS or any other condition that nuchal looks for.

Ohforfoxsake Tue 16-Sep-08 21:38:44

Yes, I thought it might mean the figure could be inaccurate.

I am wondering why are you having a CVS if the odds are so good? Is that not adding risk unnecessarily?

TotalChaos Tue 16-Sep-08 22:31:16


Deux Tue 16-Sep-08 22:51:35

I thought that the 95% accuracy meant that the FMC's tests catch 95% of the pregnancies where there is a problem, not that one in 20 had an inaccurate result? And that the 95% are in the high risk category. So only 5% of those pgs with a problem are in the low risk category.

Or is that the same thing? <sleep deprived with 6 wk old>

Am sure that's how they explained it to me.

TotalChaos Wed 17-Sep-08 09:38:36

bump. If that's how it was explained to you on the spot, sound as if my stab at an explanation was wrong then.

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