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I am not sure who instigated this thread but I have really enjoyed following it....thanks to whoever. To give our local Leeds flavour...we have ben running a LASH protocol for about 4 years or so now and have presented posters at a couple of meets. We are presently reviewing over 800 cases and hope we can make some helpful contribution to ED/CDU practise and hopefully that of our colleagues in General /Acute medicine....when we get all the data collated and publish properly this year. A few pointers of our experience: 1) Putting people in for a CT /LP strategy is sometimes VERY difficult! ...esp when the headache is a number of days old...CT no matter how advanced may not be so great...one is reliant on xanthochromia. 2) There are a variety of causes of false positvity for xanthochromia.....on the small number who have been CT negative but xantho positive...most have been false positive. True positive ...ie subsequent neurosurgical intervention required is very small....less than one hand's worth!! 3) Our CT rate is a bit higher than 5%...some of our positives had tumours or viral meningitis etc 4)We went through a phase of trying to give 'valued' informed consent and patients influencing th pathway....depends on the educational /communication skills of the patients...esp if they are from ethnic minorities....my punjabi is not bad....but not that good! 5)The 12-24 hr group of headache with a modern generation scanner and good interpreter probably will be good for avoiding an LP. 6) The spectrophotometric technique for xanthochromia is not great! Our Chem Path people tell me there is a better colormetric technique coming soon....just like high quality CTs no doubt! :-) There's probably a lot more....but as I say its a really difficult group. In the near future we are setting up on the new College website (thanks to Ruth Brown!) a subsite for Observation Medicine and will have the presentations from our recent CDU conference and copies of protocols from a variety of centre with CDUs /OUs. We hope that we can develop an interest group that can share views on issues like this and hopefully develop a webased database that can look at sharing this type of data and answer these more difficult questions once pooled. We hope to let you know more about the site soon. Kind regards Taj PS: LP position- sitting forward technique:-) --- Coats Tim - Professor of Emergency Medicine <[log in to unmask]> wrote: > Adrian, > > I think that you have confused the meaning of > sensitivity. A 98% sensitivity certainly does not > mean that 1 in 50 of patients with a negative scan > will none the less have an SAH. > I think that you have also confused the meaning of a > likelihood ratio. A ratio of 0.02 certainly does not > mean that 1 in 50 of patients with a negative scan > will none the less have an SAH. > > A 98% sensitivity means the 98% of patients WITH SAH > will have a positive scan. This is a completely > useless figure in patient decision making. We don't > want to know the proportion of patients WITH THE > DISEASE that have a positive test. As clinicians we > want to know the proportion of patients WITH A > NEGATIVE scan that hve the disease. > > Sensitivity and specificity are useless in > interpreting the results of a test. > > We need to use the negative predictive value (my > patient has a negative test, in what proportion does > this really mean that they don't have the disease) > and the positive predictive value (my patient has a > positive test, in what proportion does this mean > that they have the disease). > > For CT scanning (assuming the population prevelance > of 5%): > Positive predictive value is 99.8% (ie if CT is > positive I can say that 99.8% of patients actually > have SAH) > Negative predictive value is 99.9% (ie. if the CT is > negative only 0.1% of patients actually has an SAH) > Sensitivity is 98% > Specificity is nearly 100% > > If I want to tailor this to an individual patient > (who may have a risk that I judge is more or less > than the population average of 5%) I can use the > likelihood ratio to work out a more individual > calculation. > > BOTTOM LINE: If the CT scan is negative 0.1% of > patients will have an SAH (ie 1 in 1000). > > SECOND BOTTOM LINE: Sensitivity and specificity are > very misleading ways of describing a diagnostic > test. I have no idea why they are so often quoted. > > I feel a headache coming on, wait a minute, whats > the probability...... > > Tim. > > > -----Original Message----- > From: Accident and Emergency Academic List > [mailto:[log in to unmask]]On Behalf Of > Adrian Fogarty > Sent: 01 March 2006 01:20 > To: [log in to unmask] > Subject: Re: SAH > > > No, the other way round; I was talking patients > while you were talking > populations, I think! (see below) > > My definition of likelihood ratio: well, I was > "paraphrasing" really - and > omitted the denominator! I understand negative > likelihood ratio to be "the > probability of an individual with the condition > having a negative test - > over - the probability of an individual without the > condition having a > negative test". Now, since the latter (the > denominator) is very close to > unity, I have taken the liberty of omitting it for > clarity's sake. Hence, > when I'm told the negative likelihood ratio of CT > for SAH is 0.02, that > tells me that 1 in 50 of patients with a negative > scan will nonetheless have > an SAH. > > Yes, I gather you can also express this as > (1-sens)/spec as you suggest, but > it can be put into words; I work better with words > you might have noticed! > Again, here we have a highly sensitive test (in the > order of 95-98% > depending on the scanner etc) but an extremely > specific test (you don't > often see a bleed that's not there!). So again, you > can virtually omit the > denominator and you're left with (1-sens) which > comes back to the 0.02 > figure (1-0.98) or 1 in 50. > > I think our difficulty - as you've alluded - is that > you're "applying" this > to a 5% pre-test probability from your historical > population. But I have > some difficulty with this, as you only know they're > 5% after their scan. > Surely before you scan an individual you must > "subconsciously" think they're > much higher risk than 5%? And surely the patient > thinks they're "near 100%" > until proven otherwise, and so does their physician > who's ordering the scan? > They're thinking to themselves: "if I've got SAH, > what are the chances this > test will miss it?". And the answer's 0.02 x 100 = 2 > which is 1 in 50. > Hence, why would they then refuse an LP based on a > 1-in-50 chance they've > still got SAH? > > But I do see your broader point - and herein lies > the crux of the matter - > which is to take into account all the true negatives > mixed in there with > that one true SAH that your scan's missed. So fine, > if your population only > has a yield of, say, 50 positive scans per 1000 > patients, then you're going > to have 950 patients left over anxious for a > diagnosis, yet we know only 1 > of those is a true positive (1 in 50). That means we > need 950 LPs to find > that one positive SAH. And that's the very > compelling population argument! I > suppose it all comes down to the remarkably low > yield of 5% in your > population which rather surprised me. In other > words, if we were working in > a population where, say, 50% of our scans were > positive, then you'd have to > LP the rest, as 1 in 50 of them would have SAH. > > So there goes my rather simplistic view of > stats...it gets me into all sorts > of trouble you know. > > AF > > > > ----- Original Message ----- > From: "Coats Tim - Professor of Emergency Medicine" > <[log in to unmask]> > To: <[log in to unmask]> > Sent: Tuesday, February 28, 2006 6:50 PM > Subject: Re: SAH > > > Adrian, > We are not seeing eye to eye on this as I am > thinking about the > probabilities for an indivdual patient and I wonder > if you are thinking > about probabilities in population terms. > > The negative likelihood ratio of 0.02 is a property > of the test and is the > same for all individuals (and is independent of the > prevelance of the > disease in the population). > > The pre-test probability is what you use clinical > skill to attach to an > individual patient. An individual patient will not > have the 'population > average' pre-test probability - you will probably > find more or less worrying > features in their history. This could be from nearly > 0% (here is some > paracetamol bye bye) to nearly 100%(you have a SAH). > > The population prevelance (the 0.05 Sally Clarke > figure) is not relevant > here (in calculating the likelihood ratio the > prevelance in the population > is present on both sides of the equation and > therefore cancels out). > > I don't really agree with your definition of > Likelihood ratio as "the > probability of an individual with the condition > having a negative test". > LR-ve = (1-sens)/spec. I am not sure that I can put > this into words - it is > a mathematical number which cannot really be equated > to the sort of terms > that you are using. > > === message truncated === __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! 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