The 7 Laws of Diagnostics

March 6th 2014 is Change Day in Australia.  Change Day  is a movement promoted by individuals working within the health system. It is all about each of us Making a Pledge to do one thing (or many things) to improve the health and wellbeing of others. What pledge can you make to improve patient, client and consumer health outcomes?

I think this is an important project – it is not about hospitals, health services or government – it is all about the little things that individuals can do to make a small difference to their own ‘sphere of influence’ and do better for those with whom they come into contact.

Here is mine: “I’m pledging to promote and teach a rational approach to diagnostic testing in order to prevent over diagnosis, unnecessary and costly interventions and patient harm.”

So here is how I am hoping to do so.  Plagiarism.

I am a huge fan of Dr David Newman (Smart EM) of the Mt Sinai School of  Medicine New York City.  I was lucky enough to cross paths with David on the FOAM circuit last year and he is truly an inspirational speaker and a gentleman of profound knowledge.   So I am happy to stand upon his shoulders in order to honour my pledge.

Dr Newman has developed his own “4 Axioms of Diagnostic testing” – if you want to hear it from him check out the Smart Testing podcast.  Time well spent.

However, if you want to be slack and want it in a quick & easy format here is my “adaptation”, perversion and expansion of the Laws.  Also a hat tip to my friend Dr Anand Senthi for some of the concepts.  You should be aware that by “test” I am using a very broad definition – it includes most of the questions and clinical examinations that you perform – not just the bloods and imaging that you use every day.

Click to get examples of each Law.

This law seems self-evident. However, in our very human minds we often equate a “negative” result with “normal” with “safe” or “Fine, you’re fine! Get outta here, go home!” This is a myth. There is no such thing as zero risk. Only baseline risk. You know this – but your patient does not. They may believe that the negative ECG and troponin means they are free of any atheroma

Once again – you know this. Every question you ask, every blood on that biochem panel comes with a bell curve, a normal range and some doubt. For some we are familiar with the sensitivity, specificity, NPV, PPV, +LR, – LR blah, blah, blah…. What I see though in practice is binary thinking – if the WCC is up – I will act, if normal – I will wonder what I missed… Be aware that each piece of information is just a clue – there are few absolutes in our daily practice.

David Newman tells a funny anecdote about the man who accidentally got a bHCG test which came up “positive”. The point being – if a test is applied in the wrong context it is meaningless. This is an extreme example. But there are many subtle examples in daily practice. For example, a patient with 3 days of mild arthralgia. If you run a complete “rheumatic screen” over this patient: (ANA, ds DNA, Rheum factor, anti-CCP, uric acid, ESR, arbovirus serology, HLA B27 typing…..), then you will often come up with one or two ‘hits’. They are of course meaningless without a more specific clinical context – some sort of symptom that puts the patient in a context where a result would prompt further investigation.

Willie Sutton was a bank robber. When caught he was asked: “Why rob banks?” His answer – “That is where the money is!” When it comes to testing I believe you should start your workup with the tests that are most likely to yield a useful result. This can be tough for junior Docs – they often feel obliged to do the ‘lesser’ tests first, before going for the one that might actually make a diagnosis. As a rough rule – imaging is more useful that blood work for a lot of our common presentations. I do a lot of bedside US in lieu of bloods that will not change the situation. It is about bang for buck – but balanced against risk of radiation, potential harms etc

Thresholds come in two flavours – upper and lower. Intuitively you know this. You have all heard the mantra: only do the test if it will change your management. Rarely is this practiced! Thresholds are set by the disease: – incidence – seriousness of morbidity, mortality – characteristics of available therapies Definitions: Upper threshold – probability at which treatment is required. When will you pull the trigger on therapy. Lower threshold – the probability below which treatment would be futile or more harmful than beneficial.

A lot of the diseases that we are trying to diagnose occur in old, frail or unwell patients. In a way this is an expansion of the Lower threshold rule above. If the prognosis of the patient will not be changed – for the better or worse by your investigation – then don’t do it. Classic example would be chasing a ?DVT on a patient with terminal malignancy who has already had a few major GI bleeds.

Few thing really annoy me. One is hearing the medical students present a case and when asked: “What tests would you do?” They reply: “Routine bloods…. then …” I am not sure if there is a formal, agreed batch of “routine bloods” – let me know if I missed that lecture in Med School. Routine tests break a lot of the above rules. My guess is that there is a presumption that we will catch anything we missed on history and exam if we do the ‘routine bloods’. If you are in my ED and you utter this phrase you will be asked to justify every one of them. If you do it a second time I may spifflicate you with your stethoscope. Sure, some may be completely appropriate – and there is a valid reason for doing a lipase in the guy with bad pain radiating to his back with vomiting++. However, you should have formed this ‘pretest probability’ AKA ‘the context’ from your clinical examination and history.

OK, those are my 7 Laws of Diagnsotics. Would love to hear your additions, subtractions or examples.  As always – I am frequently wrong, so please let me know why on the comments below.




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