Adrian Fogarty wrote:
> But I still think you're overegging it, doing a "Sally Clark" as it were.
> First of all, your whole premise of the pretest probability of 5% is
> based
> on CT results. Nevertheless, let's accept for a moment that of a LASH
> population (see, you've got me doing it now!), 5% will end up having the
> target disease, so 5% is your pretest probability. But then you're
> stating
> that the negative likelihood ratio for CT is 0.02. The very definition of
> negative likelihood ratio is tantamount to "the probability of an
> individual
> with the condition having a negative test" so there's no need to
> factor the
> 0.02 by 0.05, if the figure of 0.02 has already been established as it
> were.
>
> Yes, by all means factor in the negative likelihood ratio of LP and
> factor
> that by 0.02, but that will give you those patients who have SAH but are
> negative for both CT and LP. And yes they will indeed be rare, but that's
> not who we're interested in here; we're interested in the vastly greater
> number who will be LP positive after CT negative.
>
> Maybe I'm wrong but if Tim's saying that only 1 in a 1000 negative CTs
> end
> up having SAH then maybe PB's right, we shouldn't be doing LPs at all.
> But
> from my understanding, the figures are much higher than 1 in 1000. Am I
> suffering from mental entrenchment syndrome?
>
> AF
>
> ----- Original Message -----
> From: "Andrew Webster" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Tuesday, February 28, 2006 8:02 AM
> Subject: Re: SAH
Adrian you need to remember before we do any testing the chance of
someone having a SAH is around 5-10%. We then apply a test CT. If its
negative applying the likelihood ratio of 0.02 brings the post test
probability to 1 in 1000. That is before LP. There are plenty of
calculators on the net if you want to check the figures.
"The Likelihood Ratio (LR) is the likelihood that a given test result
would be expected in a patient with the target disorder compared to the
likelihood that that same result would be expected in a patient without
the target disorder."-from the CEBM website
Andy Webster
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