>
> And Tim, you're a better statistician (and neurosurgeon) than me, I've no
> doubt, but I don't quite follow your maths approach here. If the negative
> likelihood ratio of CT for SAH is 0.02, that basically means that of
> all the
> negative CTs we get (for LASH!), 1 in 50 of those patients will still
> have
> an SAH, doesn't it? (correct me if I'm wrong here...) It seems to me that
> you're citing a pretest probability of 5% to the scan-clear patients
> (which
> is fair enough), but you're then factoring in the negative likelihood
> ratio
> of CT testing itself, giving an incredible 1-in-1000 result. That's not
> rational to my mind. You're citing a pretest probability based on the
> results of the same test, a circular argument if you like. Surely it
> would
> be more appropriate to take that pre-test probability and then factor
> in the
> positive likelihood ratio of LP wouldn't it? What I'm saying is, for
> every
> 50 patients you get coming out of the CT with a clear scan, you're
> going to
> have one who actually has SAH and that one is very likely to be picked
> up by
> the subsequent LP (assuming LP is fairly sensitive when done at the
> correct
> time etc). So it can't be 1 in a 1000, it's got to be closer to 1 in 50,
> doesn't it? (which might explain why PB's had two of them in the last few
> years...)
No Tim is saying before we do any tests ourselves the pretest
probability of the patient having a SAH is 5%. With a negative CT the
likelihood ratio of 0.02 the post test probability is 0.1% or 1 in a
1000. Which is why you need to do 1 in 1000 lp's to find a positive LP
Andy Webster
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