OK, but why did you not apply your 0.02 factor to this case? Why has the
probability only dropped from 99% to 60%? And not from 99% to 2%?
This is in danger of degenerating into a "stats" mailing list. Wow, wouldn't
that be exciting!
A
----- Original Message -----
From: "Coats Tim - Professor of Emergency Medicine"
<[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, February 28, 2006 6:33 PM
Subject: Re: SAH
Lets take this as a worked example. If your pre-test probability was 99.9%
and the CT was negative the post test probability would be 60%, and there is
no question that you would do an LP. OK, in this example you don't need the
stats to tell you what to do, but unconsciously you were using probability
theory.
I just used 5% as an example - you can pick a patient with any pre-test
probability. This is where the art and the science of medicine come
together - your clinical skill (or level of worry) determines the pre-test
probability, the science then helps you interpret what the test means for
that individual patient.
Tim.
-----Original Message-----
From: Accident and Emergency Academic List
[mailto:[log in to unmask]]On Behalf Of Andrew Webster
Sent: 28 February 2006 10:59
To: [log in to unmask]
Subject: Re: SAH
Andrew Stearman wrote:
> after reading these at work , we just had a patient with a known aneurysm
> , headache , collapse
> gsc 3 , sub hyaloid hemorrhage , my pretest probability of SAH was about
> 99.999 % , i dont see where
> we are getting this PTP of 5 % from ,that people are talking about .
>
>
The pretest probability is the chance of a normal person with GCS 15 and
an acute severe/worst ever headache having a SAH which is anywhere
between 5-10% Everyones PTP will vary. For a GP it may be less, certain
populations may be more. It is up to you to decide what your pretest
probability depending on your experience. If the patient has a known
aneurysm and GCS 3 then obviously PTP is much higher.
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