So I imagine you have just read the title of this post and thought – “Casey has finally lost it!” – or something to that effect….
Diagnostic Decoherence – it sounds like the title of an obscure SMACC lecture.
So what am I rambling on about – what is this concept? Where does it come from? And why do you care?
Let me explain.
At SMACC 2013 I had the pleasure of meeting Prof. Simon Carley – a man with a brain the size of a small planet. One evening over a beer he stumped me with a statistical problem – the Monty Hall paradox
It is a simple problem that demonstrates our often-flawed intuition when it comes to probability and decision-making. And even smart, experienced clinicians get it wrong a lot of the time. I recall walking away from that conversation feeling like a small part of the rug had been pulled from under my clinical gestalt.
The next morning Simon gave a great lecture on the real world application of statistics at the bedside – this talk was called “Wrestling with Risk” – you can see it on the 2013 SMACC podcast. Free, awesomeness.
Simon’s talk was great – a beautiful illustration of balancing risk of missing vs. over investigation and diagnosis that leads to unnecessary intervention. It is all about understanding the imperfection of our science and being able to think past the “results” of tests. We need to be thinking about what the results really mean and how they apply to the individual in your care. IN recent months Simon has teamed up with Dr Ian Beardsell [@docib] to produce the St. Emlyn’s podcast - the first few episodes go right to the heart of probability and testing in ED.
Simon used the PE workup as an example – it is one of those diagnoses where we have good data – well-defined pretest risk assessment tools and calculated likelihood ratios for our “diagnostic tests”. The problems we see with PE workup come from the common misunderstanding many clinicians have about the use of these tests. For example, performing a CTPA on a Well’s Score low-risk patient is more likely to yield a false positive over a true positive if reported as “PE”.
OK, great. We need to teach our students and trainees all these stats – educate them about test characteristics and application to various populations. If they knew all the LRs, scores and risks then they could make statistically-sensible decisions. Less harms, less false diagnoses and probably a swag of money saved.
But…. here’s the problem. We are still practicing in the Wild west of science. There are not a lot of rules, plenty of unknown unknowns and yet we need to get the job done – for most of what we do there is just not a great evidence base. Even when we do have evidence – it is often of questionable quality or cannot be applied to our patients’ particular context. Would lead many of us to EBM-nihlism or drink!
Ever since that talk I have been thinking – contemplating and trying to come up with a counterpoint. Examining the way I use probability in daily practice and searching for a good analogy in another field of science. And I think I have found it! So here it goes…. but first a quick disclaimer.
I, Dr Casey Parker, am a total physics nerd and make no apology for this. So the following rant makes complete sense to me and maybe a few other people I know! This next bit is very heavy on non-medical, theoretical ideas which I believe are a nice analogy to the conundrum posed by Prof. Carley. If this type of thing bores you to tears – stop reading now! OK, still there? Begin…
Einstein is famously quoted as stating that “God does not play dice with the Universe.” Possibly the most famous quote from the great man – and possibly the most flawed! Einstein was expressing his incredulity at the emerging quantum physics of the early 20th Century – he felt fundamentally uneasy with the idea that probabilistic mechanisms determined the nature of the universe. Specifically the Copenhagen interpretation stated that: “A system is completely described by a [probability] wave function , representing the state of the system, which evolves smoothly in time, except when a measurement is made, at which point it instantaneously collapses to an eigenstate of the observable that is measured.”
Here is where the infamous thought experiment of Schrodinger’s cat comes in (or doesn’t… if it is dead ;-). The cat exists in a state where it is both alive and dead (probabilistically) up until the point that the box is opened and an observation is made.
So when you are working up a lowish-risk PE patient – you might have a Well’s score, a modified Geneva or your trusty clinical gestalt. You plug it into MDCalc and you get a probability of a PE being present. The D-dimer is done… DOH!… it is “not negative” and you are unable to say “go home!” So you decide to make an ‘observation’ ie. a CTPA – which really just changes the probability. There is no waveform collapse, no eigenstate, no. Medicine is not like physics – there is no absolute.
But, here is the but… at some point you have to make a call. We work in the real world – you cannot go to the patient and tell them that they have a 73.2% PE. You cannot give them a 73 % dose of heparin? We need to be able to to decide who gets treated and who does not. This is analogous to the classical-quantum divide: we know that particles are composed of probability waves, yet when you look around you – you see objects, collisions, hard surfaces all of a predictable nature. When your patient falls – their radius breaks, when the clot sticks, they get ST elevation. So how can we reconcile this in our practice.
For the vast majority of diseases or problems that we see in the ED there are no numbers, no starting position – we are sailing in uncharted waters and would be lost. I say “would”. Because we do have our own personal onboard navigation systems – we all carry a rich library of experience and “gut instinct” around in our heads. This is what we call “Gestalt” – that oft spoken German word which is rarely understood. So how does this work?
Back to the quantum world… For most of the 20th century theoretical physics was stuck with a big headache. On one hand there was a great theory that explained the big things – Einstein’s relativity. And then there was the new kid on the block – quantum theory which used probability and wave equations to describe and predict the tiny world of the fundamental particles. However, there was a demarcation dispute. At some scale the particles had to start behaving like classical physical objects. But there was no way to marry these two perfectly functional theories in such a way that made any sense. This is where Decoherence comes in.
When we take a history and examine a patient – we are building up a set of probabilistic waves. Each piece of data increases or decreases the amplitude of the wave. Each wave carries the probability of a diagnosis – and to reach the threshold of action / reality / or the “treatment threshold” the wave must break over the wall. Stick with me – it gets clearer soon.
Simon has beautifully described how various factors create waves of various amplitude in the work up of chest pain. Check out his SMACC Gold lecture from 2014 - on risk factors. When we take the history from a chest pain patient – we know that the typical atherosclerosis risk factors are poor predictors of acute coronary syndromes. In other words – they produce only small waves, they are subtle ripples in the ACS pond. In order to get over the threshold – we need a collection of waves which positively interfere with one another in order to make it to an “actionable” probability. Chest pain is easy – we have good data to measure the waves and the amplitude of the threshold. Alas, when you are floating on the “non-specific abdo pain pond” the waters are choppy. Lots of waves interfering with one another – leaving us with murky water. That is why these patients seem tougher to sort out. And by the way – no matter what the Surg Reg tells you – the white cell count is merely a ripple on that pond!
Now of course, some of our “tests” create big waves – think about “thunderclap headaches”, “shingles rash” or a “3 mm of ST elevation pattern in II, III and aVF”. These features create waves that easily carry us over the “action threshold”. This is the stuff we all learn in Med School. Sometimes it is wrong – for example – Framingham risk factors and ACS – no real wave in the ACS setting. However, more often it is just plain messy.
When the quantum physicists look at probability waves – they isolate a couple of particles and do complex probability wave calculations – very smart. But if you try and do this with more than 3 particles (waves) at a time – the numbers get crazily complex – it is just too messy. And yet we all do this intuitively on a daily basis. One of my mentors believed in “Diagnostic Triads” – the idea that many diseases could be reduced to 3 symptoms – this is an example of three simple probability waves interfering to create an ‘eigendisease’ -e.g. fever, jaundice plus (R)UQ pain = cholangitis. Unfortunately during our daily grind in the ED the act of taking a clinical history and exam is an infinitely complex assessment in which we are constantly integrating new data and probability waves into the mix. At some point the waves may coalesce into a meaningful reality – and this is what a quantum physicist might describe as Decoherence. The waves become a real thing – something that we can name, refer, incise or thromobolyse.
So in summary – I agree with Simon. [ ‘twould be daft not to! ] We are probabilisticians rather than diagnosticians. However, the math is far more complex than our textbooks and teachers would have us believe. Medical students have always struggled with the complexity of clinical diagnosis. There are just too many variables for even the brightest minds to integrate into a cohesive model of what is going on in most patients. Of course – spending more time, doing more tests can give us more data points and help us “realise” a diagnosis. And as we gain experience we learn to recognise “disease scripts” – patterns in the waves that we recognise as “a thing” before they reach the threshold.
Now – one might read all of this and get a sense of nihilism. It all just seems too complex and hard. But I think there is a way forward. Here is what I believe we should do:
1. We need more research to define the probabilities – clinical medicine is full of dogmatic belief in our clinical skills. E.g. “the chest is clear – therefore no pneumonia….” In reality the negative LR for auscultation in pneumonia is close to 1.0. Utterly useless! By knowing and teaching the reality of our skills we can do better, make more reasoned decisions.
2. We need to teach probability to Medical Students – personally I am a fan of the Bayesian approach and teach likelihood ratios as the most easily applied stats to keep in the back of one’s head. These are more intuitive than sensitivity or specificity. They tell you more about the “test” and can be applied directly tot he individual patient in front of you.
3. We need to change the language that we use when we talk to our patients. A while back I defined the Zeroth Law of Diagnostics – “there is no such thing as Zero risk”. And I believe that we ought to fess up to our patients – certainty is a rare beast in Medicine. We should tell our patients this. We can have a reasonable belief that they do not have a PE, or that they do have ‘smouldering diverticulitis’ – however, there is a real chance that we are wrong. If things change, or they get new symptoms [waves – new waves ] then they need a review and rethink. The catch phrase for this is = “shared decision-making”… or as we in GP-land call it : consultation. The patient is the one with the disease [or not] and they take the risk.
4. We need a bit of Dogmalysis – there are a lot of pseudo axioms and long-held truths in our game that need re-examination. David Newman has produced a nice series of podcasts on this topic – check it out at SMART EM.
5. We need to develop a healthy insight into the way we think – I believe the buzz word is metacognition. Understanding the sources of error and recognising the false waves will help us to make better, more accurate decisions – and to do better by our patients.
Ok. That was quite a ramble. Back to something more concrete soon! Or is that concrete really just the summation of a pile of waves of probable concrete particles ?
Let me know what you think.
Andif you want to read more about Decoherence in quantum physics – I recommend reading this quick review paper. For more reading check out the Decoherence website here. Or just go outside and get some sun
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