Now I'm intrigued Paul. I'd like to know exactly what you tell them 
(seriously). In this part of the world we're very black and white. If your 
CT's negative, you still have a reasonably significant chance of having had 
an SAH, particularly if your history's good and the CT's been delayed by 
several hours. So we tend to just tell those people "you need an LP 
now...turn over"! No argument.

So what's all this touchy-feely informed consent about then? I mean, what 
figures do you give them? Surely if you tell them that they've still got a, 
say, 5% chance of having had a bleed, but a negative LP will reduce that to 
virtually nothing (see below) then who's going to refuse that? So why even 
have a discussion is what I'm trying to say.

And Tim, you're a better statistician (and neurosurgeon) than me, I've no 
doubt, but I don't quite follow your maths approach here. If the negative 
likelihood ratio of CT for SAH is 0.02, that basically means that of all the 
negative CTs we get (for LASH!), 1 in 50 of those patients will still have 
an SAH, doesn't it? (correct me if I'm wrong here...) It seems to me that 
you're citing a pretest probability of 5% to the scan-clear patients (which 
is fair enough), but you're then factoring in the negative likelihood ratio 
of CT testing itself, giving an incredible 1-in-1000 result. That's not 
rational to my mind. You're citing a pretest probability based on the 
results of the same test, a circular argument if you like. Surely it would 
be more appropriate to take that pre-test probability and then factor in the 
positive likelihood ratio of LP wouldn't it? What I'm saying is, for every 
50 patients you get coming out of the CT with a clear scan, you're going to 
have one who actually has SAH and that one is very likely to be picked up by 
the subsequent LP (assuming LP is fairly sensitive when done at the correct 
time etc). So it can't be 1 in a 1000, it's got to be closer to 1 in 50, 
doesn't it? (which might explain why PB's had two of them in the last few 
years...)

If I'm way off the mark just explain it to me like a six-year-old; that 
usually works for me...

Regards

Adrian


----- Original Message ----- 
From: "Paul Bailey" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, February 27, 2006 4:19 PM
Subject: Re: SAH


> Thanks Tim,
>
> I think it's pretty simple.  You explain to the patient what a risk averse
> approach is, and see if they want to pursue it.  If they don't you 
> document
> it.
>
>
>
> I find it strange that some believe that LP is a 100% sensitive test. 
> There
> are *very few* 100% sensitive tests in medicine.
>
>
>
> I look forward to reading your article.
>
>
>
> PB
>
>
>
>
>
>  _____
>
> From: Accident and Emergency Academic List
> [mailto:[log in to unmask]] On Behalf Of Coats Tim - Professor of
> Emergency Medicine
> Sent: Monday, 27 February 2006 11:32 PM
> To: [log in to unmask]
> Subject: Re: SAH
>
>
>
> Paul,
>
> I like your approach to a discussion about risk with the patient. I think
> that it is the right thing to do, but there are some very entrenched
> opinions out there - the reviews that I recieved from well know journals
> when I submitted the risk paper showed how much dogma is still there.
> (Comments such as "This is dangerous and shoudl be rejected"). Bitter? 
> moi?
>
>
>
> The radiologists perpetuate the dogma here by adding the phrase "Lumbar
> puncture required" to the end of the report for every CT scan for 
> headache.
>
>
>
> In this context I think that you are sensible to carefully document your
> interaction with the patient - I have not yet had the courage of my
> convictions to make this my routine practice. Maybe I should. I would be
> very interested in the list's views.
>
>
>
> Tim.
>
>
>
>
>
>
>
> ge-----
> From: Accident and Emergency Academic List
> [mailto:[log in to unmask]]On Behalf Of Paul Bailey
> Sent: 27 February 2006 11:57
> To: [log in to unmask]
> Subject: Re: SAH
>
> 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
> [mailto:[log in to unmask]]On Behalf Of Paul Bailey
> Sent: 25 February 2006 03:44
> To: [log in to unmask]
> Subject: SAH
>
> I 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 Bailey
>
> Emergency Physician
>
> Western Australia
>
>
>
>
>