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I did apply the 0.02 LR-ve in this case. The relationship between pretest prob, likelihood ratio and postest prob is not a straight multiplication - you need to use the nomogram. There is a cute version of the likelihood ration nomogram on the Oxford Evidence Based Medicine Website at http://www.cebm.net/nomogram.asp Hours of fun. Tim. -----Original Message----- From: Accident and Emergency Academic List [mailto:[log in to unmask]]On Behalf Of Adrian Fogarty Sent: 01 March 2006 00:26 To: [log in to unmask] Subject: Re: SAH 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. This e-mail, including any attached files, may contain confidential and / or privileged information and is intended for the exclusive use of the addressee(s) printed above. If you are not the addressee(s), any unauthorised review, disclosure, reproduction, other dissemination or use of this e-mail, or taking of any action in reliance upon the information contained herein, is strictly prohibited. If this e-mail has been sent to you in error, please return to the sender. No guarantee can be given that the contents of this email are virus free - The University Hospitals of Leicester NHS Trust cannot be held responsible for any failure by the recipient(s) to test for viruses before opening any attachments. The information contained in this e-mail may be the subject of public disclosure under the Freedom of Information Act 2000 - unless legally exempt from disclosure, the confidentiality of this e-mail and your reply cannot be guaranteed. Copyright in this email and any attachments created by us remains vested in the University Hospitals of Leicester NHS Trust.