Emergency Medicine, a risky business part 1. You are not a diagnostician.

Time to think about whether you are really an emergency medicine diagnostician…

Are you a diagnostician? Of course you are! Why do I even need to ask?

Well, to be honest I’m no longer very sure to be honest. When I speak to friends and colleagues in emergency medicine and ask them to describe themselves then ‘diagnostician’ is a word that is commonly used. Now I know what they mean and so do they, but is it the right word? Making a diagnosis is perhaps not quite as straightforward as you might think, and if diagnosis is quite as straightforward as we think then perhaps we are not diagnosticians.

[Ed – Eh? I’m getting lost in the semantics here – give us an example]

So, let’s think about something really easy. Have a look at the ECG below and tell me what the diagnosis is. The patient is a 54 year old man with chest pain.

So, I’ll bet that you said inferior MI didn’t you. You did, I know you did because I’ve had said the same thing too, but we would both be wrong, well possibly….

The first reason is that inferior MI is a politically incorrect diagnosis according to the politically correct guide to cardiology.

Secondly, and leaving the PC brigade behind, when we look at this ECG we come to a conclusion that it is indeed an MI but yet we also know that there are many ST elevation mimics such as LV aneurysm. In this case we know that we should still start therapy and get the patient off to cath lab but there is a possibility that we will be wrong (about 5% of the time in fact). So what we are doing is attaching a label (in this case MI) to a probability.  So it’s probably an MI, so let’s treat it as an MI, and overall patients (as a population) will be better off.

Great, but that’s all a bit picky isn’t it? Does this really matter? Well in the example above perhaps not as we are going to treat on the basis of a high risk, so we have decided a diagnosis and we are going to treat. Now we know that this presents problems for some patients as the therapy for things like MI (and stroke) are risky in themselves so every so often we will harm a patient through a known complication of a therapy for a disease they do not have. Worth stopping and thinking about that one for a moment, it means that if we assign therapies on the basis of a probability harm will happen, sorry harm WILL happen.

The difficulties also arise when we consider what we might do in the exclusion of disease because the same problems arise. Let’s think of another example where we are seeking to exclude a diagnosis, if we are talkind about probabilities when ‘ruling in’ a diagnosis, then do we get similar problems when we ‘rule out’?

The diagnosis of the moment seems to be PE with loads of posts and debates pinging around the blogosphere, and I’m sure that you will seen a number of rule out strategies for the management of ?PE patients. I’ll also bet that in the majority of cases the implication of the rule out strategy is that if the patient is negative for investigation then the patient does not have a PE, but this is not true. Just as it’s not true for most rule out MI, appendicitis, UTI, renal stone, Sub arachnoid bleed protocols. What we are actually doing is moving patients from a pretest probability where we are worried down to one where the risks of pursuing further investigation outweigh the benefits. This is one of the reasons why I like the way Scott Weingart wrote the rule out PE pathway at EMCRIT. The one from Scott Weingart is not typical in that respect as the end point is ‘stop work up’ as opposed to PE ruled out – because it’s not. In reality the sensitivity of most PE rule out strategies is in the 90’s but they are certainly not 100%.

The obvious result of this is that if you use a typical rule out protocol you are really using a probability protocol and therefore you must be missing something. Let’s take a really good R/O protocol for PE that has 98% sensitivity, – that’s fab and 98% is regarded by most people as a SnOut, but what we now know is that a sensitivity of 98% means that we miss 2% of patients with disease. That’s 1 in 50 folks, with the implication that you will be missing a lot of serious disease in your career……or does it? (more of this later)

Now that you are worried that you’re not a diagnostician at all, just a doc playing the odds and gambling with your patients life it’s time to think about the implications of being a probablestician rather than a diagnostician, and perhaps why as clinicians we like more certainty than we typically justify. That’s for part 2.

vb

Simon Carley

 

PS. Since I made the word probablastician up I’m not sure how to spell it.

PPS. In future articles we will take about diagnosis and diagnosticians, but remember that things are rarely as certain as they seem.

 

For all posts in this series click here

Risky Business Part 1 You are not a diagnostician.

Risky Business Part 2 What does a test result really mean?

Risky Business Part 3 Is it acceptable to miss some diagnoses?

Risky Business Part 4 How many patients do we miss (and does it matter)?

Risky Business Part 5 Does a correct diagnosis mean that the therapy will work?

Risky Business Part 6 How many steps to disaster?

Risky Business Part 7. Risk Proximity

 

Before you go please don’t forget to…

 

 

 

14 Comments

  1. dreapadoir

    With respect to PE work up and the PERC rule, one of the problems with it (and believe me I think it’s great to use) is that it’s actually called “rule out criteria” when in fact the whole point is that it is “achieve a miss rate of <2% in order to avoid harm by over investigation and treatment" – maybe it should have a different acronym???

    I always think that my job is to resuscitate and make dispositions. The diagnosis is a bonus which often makes both of these easier, but the nature of our job is that we MUST accept uncertainty and balance the probabilities. Liking the new look!!

    Reply
  2. Simon Carley (Post author)

    Cheers, I think you are right. One of the future posts in this series will explore how and why we use language in the way that we do. Not to spoil the party, but a lot has to do with fact that we don’t enjoy uncertainty.

    vb

    S

    – and if you are reading this pop over to Domnhall’s site at http://underneathem.com/ which is looking awesome.

    Reply
    1. dreapadoir

      Thanks Simon. I agree we don’t enjoy uncertainty but we must accept it sometimes! Terry says hi, and told me about your hungover hypnotism!

      Reply
  3. Damian Roland

    Looking forward to future articles.

    To be honest though although I take your point I disagree. I do believe ED physicians are expert diagnosticians however they are not diagnostic experts.

    They have the ability to judge the risks of a variety of diagnoses picking a few chief contenders (as you explained in the article and therefore expert diagnosticians) but rarely ever arrive at final definitive diagnosis (because they now how risky that is).

    As I say looking forward to more articles…

    and further semantics

    Reply
  4. Chris Nickson

    I think the point needs to be made that all diagnosis is probabilistic.

    Anyone who thinks otherwise is kidding themselves (unless they’ve chopped out the tumour and looked at it under the microscope… although even pathologists are subject to subjectivity).

    ‘Rule out’ simply means reducing the the probability of a disease to a a low enough level that we can live with. The term is not great given that we humans have a natural proclivity to wanting to turn uncomfortable certainty (’50 shades of grey’) into absurd certainty (L. Ron Hubbard’s dianetics perhaps…).

    Great summary of the basics of diagnosis and test thresholds over on ALIEM:
    http://academiclifeinem.blogspot.com.au/2012/09/diagnostic-tests-asking-right-questions.html

    Great post!

    Chris

    Reply
  5. Casey

    Hi Simon
    Pretest prob -> likelihood ratio -> post test prob
    Sounds simple enough but requires a lot of thinking! David Newmans laws of diagnostic tests are gold, but…. There is always a but.

    We are not great at estimating pretest probability – this may be improved with experience. Most junior docs use estimates either way too high or low to allow realistic output from the above Baynesian analysis .
    Then there is the problem of thresholds – some doctors might be happy with a 2 % for PE – however some patients will want a 99.9%. Balancing reality of medical science with expectations is a skill – we should teach this in med school?
    Casey

    Reply
    1. Simon Carley (Post author)

      Fantastic stuff Casey, I’m going to talk about some of the stuff you talk about in future posts so will keep my powder dry for now.

      In answer to the second question then the answer is absolutely yes. McMaster in Canada have been doing it for years. Personally I try and teach it at foundation (1st year qualified) doc level. In EM I think this is absolutely core knowledge. We claim to be diagnosticians so we really should understand diagnostics.

      S

      Reply
    2. Chris Nickson

      The 2% threshold for PE is based on Kline’s calculations that if you pursue the diagnosis when the pretest probability is lower than that threshold, you will do the patient more harm than good (treatment of false positives, CIN, bleeding, the fact that some patients will not respond to treatment anyway etc etc). So if a patient wants greater than 98% certainty, the onus is on the doctor to explain to them that they shouldn’t.

      Chris

      Reply
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