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Greetings from a lurker...
To interpret your LP results for xanthochromia you need to know how your laboratory measures it. You can be sure that all academic research in this area has been done using spectrophotometric CSF anaylsis and that your laboratory most likely uses visual interpretation (i.e. the lab technician spins the sample, holds it up to a white card and decides if it's yellow or not!).
Given that visual interpretation of CSF for xanthochromia is unlikely to be either sensitive or specific enough to be of use in the subset of patients with ?SAH who have a negative CT (16 or 64 slice), perhaps we should only consider doing them if the sample will be spectrophotometrically analysed?
This was the subject of an article in July's Annals of Emergency Medicine.
David Menzies
Emergency Medicine Registrar, Dublin
From: Paul Bailey <[log in to unmask]>
Reply-To: Accident and Emergency Academic List <[log in to unmask]>
To: [log in to unmask]
Subject: Re: SAH
Date: Mon, 27 Feb 2006 19:57:22 +0800
Tim,
Thanks for the reply.
1) Role of LP when CT negative - depends on the pre-test probability of SAH. About 5% of the scans that we do for LASH show SAH (so we know that Emergency Physicians's threshold for CT scanning is about a 5% risk - you can work out your own threshold by looking back at what proportion of the scans you have recently ordered are positive). With this 5% pre-test probability and a negative likelihood ratio of 0.02 for CT scanning the post-test probability of SAH means that it is likely that about 1000 LPs will need to be performed to find each CT negative SAH.
I know that’s what the books say, but I don’t know anyone who has had a CT negative xanthochromia positive patient end up having a neurosurgical procedure. Just because they have a positive LP does not mean that it is of any particular value. I have had two such patients in the last few years (more than my share if the 1:1000 LPs above is to be believed) and both had negative MRAs.
(Assuming that CT is as good a test in low probability patients as in high probability patients). (Article with the details due to be published in the European Journal, March edition). As CT scanning is a good test for SAH, we maybe should be talking to our patients about their post test probability, their attitude to risk and whether they wish to have any further investigations (LP is probably not to be undergone lightly).
I already do this. I attempt to put the likelihood of the disease, including the tiny likelihood of death in context of the risks of LP for the patient. ie true informed consent (or as close as you can get when operating in the evidence vacuum). In the end it get’s down to the patient’s approach to risk. About 25% will elect to have an LP – these are the risk minimizers. 75% elect not to – these are risk tolerant patients.
2) CT angio has limited resolution and is answering a different question from a LP. If you want to know if there is an operable structural abnormality CT angio may provide the answer, however the question usually is 'has the patient had an SAH'. If I was a patient I think that I would want to know if I had suffered a SAH, even if no structural abnormality could be found. (Maybe I would give up my hobby of bungee jumping and be just that bit more appreciative of life!).
3) The influence of new generation CT scanners is interesting - you are correct about technology running ahead of us. All of the published data (the 98% sensitivity etc) seems to be from more than 5 years ago - therefore before the current generation of multi-slice scanners. How does this alter the negative likelihood ratio? Not sure, but it probably makes a CT scan a more sensitive test.
We risk stratify and accept 'low risk' rather than 'no risk' in many areas of emergency care (a PE can be just as fatal as a SAH). It is therefore a bit anomalous that we strive for 'no risk' in SAH.
My views on investigation of LASH come with a health warning - the dogma that every patient who has a CT for LASH must have an LP is very well entrenched. There is insufficient data to support any other approach, so deviate from the current 'norm' at your medico-legal peril. However, I will bet my hat that in 10 years time we will be risk stratifying and limiting the number of LPs that we perform.
I do not believe that this is true. I explain the usefulness of LP to these patients, in front of a family member. And I am honest. I get them to discuss it, and then get them to give me an answer. I should also state that I probably do more LPs than any other consultant in my ED as I don’t mind doing them and often end up ‘rescuing’ my colleagues who have failed. I’m not averse to doing LPs when the patient wants to.
If *the patient* decides not to have a LP, I insert the following phrase into their medical records (and I write electronic notes so they all get the same statement):
Explained to the patient, in the presence of (family member X) that SAH has not fully excluded until a LP has been performed and is negative. Explained the risks of missed SAH including a small risk of persistent vegetative state and death. The patient has elected not to undergo LP.
The simple fact of the matter (I believe) is that you cannot force the patient to have a procedure to which they do not consent. Whether, of course, this would stand up in court is another matter and I am not a lawyer.
Would you have any of you let a PGY2-3 doctor do an LP on you in a similar situation?
Paul
Tim Coats.
Professor of Emergency Medicine.
Leicester University.
-----Original Message-----
From: Accident and Emergency Academic List [mailtoo:[log in to unmask]]On Behalf Of Paul Bailey
Sent: 25 February 2006 03:44
To: [log in to unmask]
Subject: SAHI am beginning to wonder if I am still subscribed to the list as I haven’t heard much from you all of late.
I wanted to bring up the topic of subarachnoid haemorrhage.
I find myself, as time goes on, becoming increasingly interested in patient safety, uniformity in the delivery of emergency medicine (or complete lack thereof), the concept of ‘risk assessment’ as the core of our specialty, the concept of acceptable miss rates for particular conditions, and finally the approach to low prevalence high mortality conditions such as SAH. In a lot of imaging related areas we have the additional problem of the wavefront of imaging technology being significantly ahead of the literature on most subjects – eg CTPA for PE and the role / sensitivity / accuracy of multislice CT – creating an evidence vacuum for a lot of the things that we do.
With that out of the way I have been thinking a lot about SAH recently.
In particular, two things:
the role of LP when a high quality (ie no movement or artifact issues) multislice CT is negative.
Why we aren’t just doing CT angiograms on everyone (and perhaps obviating the need for LP) given that this is what happens in a lot of centres when xanthochromia is positive.
Out of interest, I am sure it occurs, but no-one in my institution has seen a 16 slice CT Head negative xanthochromia positive patient actually end up having a procedure – ie the CT angiogram / MR angiogram is always normal in this situation and no-one can really figure out where the blood came from.
So, I’m opening it up to the collective wisdom of acad-ae-med. What are all of your thoughts on the matter?
Kind regards
Paul BaileyEmergency Physician
Western Australia
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