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.




  1. Ah yes, “routine labs.” I heard Bill Paolo speak at #AAEM2014 over the weekend and he put into words something I had a hard time expressing for many years: “Routine ‘belly labs’ are clinical data dredging. You’re throwing a lot of tests against a wall hoping something sticks.”

  2. An excellent summary, thanks Casey. Over-testing is a huge issue in medicine, particularly as one adjusts from training by sub-specialists into a generalist career.

    As an aside, I enjoyed reading the word zeroth for the first time in my life. I have just added it to my computer’s spell checker, to negate any future automated attempts to contest it with a red underline…I’m going to use it in public at least once in 2014.

    • Hi Justin FYI. There is a Zeroth cranial nerve. Ours is tiny, in roof of the nose
      In other animals it is there as a large structure that detects pheromones – all about sex !

    • John Buckley says

      Thanks Casey. The only justification for any ‘routine testing’ is as baseline and even then it needs thinking behind it. Justin, I am ahead of you – I have zeroth for the zeroth time every year (bugger – I just blew 2016 big time!)

  3. This is a great little article!

    The macro view, i.e., the doctor’s role in the patient’s health within the context of their life can often by overshadowed by the focus on the technical details of disease, investigations and therapeutics. As per the above second and fifth “law”, test results can only be understood if you are asking the right question – and the “right question” is one where the answer has a meaningful impact on the patient’s health within their context.

    One thing that I find that students/registrars/doctors often don’t have a good conceptual understanding of is that post-test probability (i.e., the predictive value of the test result) is intrinsically, and inescapably linked to the pre-test probability. This is what often leads to false reassurance from a negative result, and over-investigation/intervention of a positive one (e.g., your description of “binary thinking”). You may have seen this before, but this is a simple qualitative approach to Bayes’ Theorem:

    • Thank you for that great link. I really struggled to get the notion of pre-test probability (and the value of scoring systems for estimating pre-test probability) to my first-term intern last term.

      My new goal for interns is, by the end of the term:

      – recognise and appropriately escalate sick patients
      – be able to explain and apply Bayes Theorem
      – have read the House of God

  4. Great set of rules Casey.
    I like the point about baseline risk in the Zeroth law – I don’t think we emphasise this enough to our patients.
    I may also take up the challenge to use Zeroth in public .

  5. Ron Cassano says

    Ah Casey! I think your 7 laws are commendable , but in the real world of Broome, as you know, possibly the most important question is : will the test make any difference to outcomes?

    Talking about bang for buck: is it sensible or morally correct spending, say $50000 ( minimum in transport, investigations, procedures, follow up) in anyone who will never take their medication, will continue to smoke and drink to intoxication daily, has uncontrolled diabetes and other vascular risk factors?

    I think not, in the context of a struggling public health system where that money could be so well spent on individuals willing to take any responsibility for their health!

    So, yes, think carefully before you order any test, and especially before organising expensive treatments and transport to tertiary centres : is this going to really help the patient, or am I doing it out of robotic protocol or to make myself feel better?

    I certainly realise that Govts hold the key here to get a bigger bang for their buck, but clinicians should be at the forefront of suggesting/implementing change.

    This may seem harsh, but all change can be hard to accept.


    • Ah Ron – the perennial practical ethics of Broome. Yep, this debate has been smouldering in our part of the world for as long as I remember.
      More and more I am having long discussions with patients and family about the reality of what testing / treating / transferring really means of them in a real-world sense.
      I feel a lot better about these decisions if they are made with the patient – where possible. Of course, not always the case. Often I find we are all in agreement about what they want out of their care.

      What do I teach the JMOs when it comes to embarking on a treatment plan that you know is “doomed to fail” for many tough social and cultural reasons?
      I reckon everyone should get one red-hot go at the ‘best possible care’, with all the assistance and supports / practical aide I can get of them.
      However, if the patient decides to refuse / decline / not follow through – then I do not chase them anymore. I keep an open door policy in my head – they may change their mind / priorities in the future – so no bias on my part based on previous behaviour.


      • Ron Cassano says

        An excellent response and a great teaching strategy; I agree fully with your ideas. Perhaps they need to be deseminated more to SMOs as well, especially to short term locums who often fail to see that the 4th transport to Perth (after 3 previous unsuccessful times) is in essence an unjustifiable huge expense, (in a struggling public health system) ,and is often pushed onto the unwilling patient to make the doctor feel better.

        I know that 1 of the problems is the short ward SMO turn around time, and another is the learning of JMOs of different medical “styles” which often leaves them confused as to what is to be done in a given clinical situation ; this can’t be good for their futures.

        Leaving aside the politics of “correctness” and “use all funding available or you lose it” ( even on very expensive items which produce no outcomes), I think JMOs have an understandable expectation of consistency. As necessary as it is learning when not to order a DDimer, perhaps a significant amount of teaching time should be about how to deal with the ethical, real world problems we actually come across .


  6. Tricky.

    I would be uncomfortable in NOT offering best care on the basis of distance, cost of transfer or likelihood of recidivism.

    Not really fair to suggest that this is a frivolous decision based on protocol or self-appeasement….the current medico-legal climate would make it hard to defend NOT offering a treatment based on distance or lifestyle.

    Dont get me wrong, am more than happy to make the hard decisions and decline inappropriate tests-transfer, palliate locally where appropriate.

    But given the way many tertiary ICUs seem to still flog octogenarians in the last few days of their long life, I think that this is a decision we need to have as a society, not castigate the poor rural generalist for “costing the system money”

    The question is, will Govt and society step up to the plate? Or continue to dilly-dally around edges with schemes such as a paltry $6 copayment for GP bulkbill services, not address wider system wastage and cost shift, nor hard decisions

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