Scan dilemma

[Inanna]

JUst an aside I agree the cut offs can be confusing - the figures given by JJe are not necessarily those used of all practitioners.

A great number of foetal medicine obs will use the amnio risk rate as their cut off. So the risk of foetal loss with amnio is around 1:250 (again this depends on the practitioner - I personally know one with a rate of 1:350 - it also must be remembered that these rates are using figures of foetal loss in the 4 weeks after amniocentesis/cvs. If you took me as an example I have lost 2 babies at around 16 weeks had I had amniocentesis my babies deaths would have been put on my practitioners stats...)

So if the amnio risk rate is 1:250 then any risk figures less than that are considered for amnio. Some practitioners use 300 so anything under 300 then the woman/family is counselled with regard to further testing. This is because the risk of the baby having a chromosomal disorder equals the risk of the loss of that baby through amniocentesis.
I fear I am sounding confusing!

[jja]

I stand by my comment that NT U/S married with bloodwork done by a well recommended foetal medicine obstetrician is extremely accurate.

I don't know where you got your statistics from jja, but my pamphlet on Ultrasound Screening for Down Syndrome (obtained from Women's Imaging Centre Freemasons) says that the combined ultrasound with first trimester maternal serum screening has a detection rate of Down Syndrome of almost 90%. I would consider this fairly accurate and certainly worthwhile as a method to help you decide whether to access further invasive testing. I agree with Flowerchild.

I definitely agree its a worthwhile method to help you decide whether to do more invasive testing, and that is its purpose.

But I think you might have misread what I wrote sorry. My figures were for the NT test accuracy *alone*, whereas your figures are for *combined* NT with serum screening which indeed improves the screening accuracy. The only reason I mentioned it was Flowerchild's original post just said NT was extremely accurate without mentioning the combo with serum screening, and I didn't want anybody to get the wrong idea about it. Also I suppose it comes down to semantics and what your definition of extremely accurate is, because personally I would never even describe the combined screening that way but thats just me

Even in the combined tests with a circa 85-90% detection rate, the false positive rate is still in the 2.5-5% range. This means that up to 1 in 20 people screened will get a "positive" screening result, but as we all know, the actual occurance of Downs is not 2.5 to 5% of pregnancies. So the usual result of a positive screening test is actually a normal CVS result - so I can't call that extremely accurate. The combination screen is only "accurate" in the context of being a screening procedure, just like Flowerchild said. As a diagnostic test its crap - not accurate at all - well its not really a diagnostic test I know, but hopefully somebody will get my point.

To me a diagnostic test like CVS is "extremely accurate" because it gives you a definite result and is extremely rarely wrong, but a screening process isn't because its just a way of calculating your odds by comparing your results against statistics - sort of like using a form book to calculate odds at the horse races. And if using odds was even just a bit "accurate", we'd all be millionaires from correctly picking all the horse races by now Except for me because I'm not much of a betting person.

Its just a matter of semantics and I'm probably being too picky Sorry...