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  • Diagnostic errors (delayed, wrong or missed) are a major cause of harm in clinical practice

    Diagnostic error rate in medical wards is 10-15% and 1 in 100 errors results in harm (Graber et al. BMJ Qual Safe 2012). This results in harm to patients and health professionals. Additionally, it wastes precious resources. The causes of diagnostic error can be broadly divided into two categories - cognitive errors and system errors. Poor clinical reasoning and cognitive bias play key roles in cognitive errors.

Cognitive Bias Books

  • Clinical Reasoning

    Excellence in clinical reasoning is a key skill for clinicians, to consistently provide safe, high quality and timely care (Monash Health Strategic Plan 2023: Guiding Principle 1).

    Clinical reasoning describes the thinking and decision-making processes associated with clinical practice (ABC clinical reasoning 2007 p1). It primarily involves analysing and synthesising information to make trustworthy judgements.

    Examples of clinical reasoning include: 

    • Creating a short list of differential diagnoses, the most likely diagnosis and serious conditions that need to be excluded 
    • Weighing up the value and risks of specific investigations, to select the most useful and safest i.e. the most appropriate investigations
    • Considering the potential advantages and disadvantages of different treatment options, including applying evidence-based information to a specific patient and incorporating the patient’s preferences  

Further reading:


Clinical reasoning occurs via two common pathways, according to dual process theory. 

System 1 - intuitive thinking 

This occurs when you ‘just know the answer’, such as a spot diagnosis. It is based on pattern recognition, founded on previous experience and knowledge. Learning more about evidence-based symptoms and signs is valuable. For example: 

  • PE without symptoms is very rare (<1%)    
  • PE without any of the following risk factors is very rare (<1%) :
    > 65y| immobilization| malignancy| hypercoagulability| excess oestrogen| previous VTE
  • Almost all (94%) of patients with PE have one of the following symptoms:
    chest pain | sudden onset SOB | fainting/syncope | haemoptysis

This type of thinking is rapid and easy. Our brains predominantly favour this method, as it enables us to function efficiently in our complex world. If we had to use Type 2 thinking all the time, we would take much longer to complete any task. However, ‘jumping to conclusions’ is risky, as it is more prone to error. Mistakes are more likely in those with less expertise or in anyone who is overloaded or working at night. 

In clinical practice, most diagnostic error is due to clinical reasoning problems. 

System 2 - analytical thinking

This occurs when you actively process different elements that contribute to making a decision. One example is listing key causes of breathlessness (maybe using categories such as site: heart / lungs / chest wall, or speed of onset: sudden / hours / days) and considering how likely each one is in a specific patient, based on their characteristic features.

This type of thinking is slower and more arduous. 

Clinical decision tools support analytical thinking. Evidence-based algorithms can be used to risk-stratify patients, for example CHA2DS2-VASc score for the risk of stroke associated with atrial fibrillation, to assist with decision-making around anticoagulation.

Similarly, mnemonics, rules of thumb and checklists can foster analytical reasoning. 

Some examples include:

  • Mnemonic: DRSABCD for immediate management of a sick patient - Danger; R etc
  • Rules of thumb: Abdominal pain in a young woman is ectopic pregnancy until excluded.

Common types of cognitive bias

Type Description
Search satisficing Once one diagnosis is found, we stop looking for anything else.
Posterior probability error We tend to assume that current symptoms are caused by a known medical condition.
Availability bias We are much more likely to think of a diagnosis if it is within our speciality, we have just seen someone else with it or have been reading about it.
Outcome bias The desirability of an outcome influences our thinking, for example a surgeon may conclude that sepsis in a patient is due to pneumonia rather than an anastomotic leak following surgery.
Commission bias We prefer to do ‘something’ rather than ‘nothing’ (or watchful waiting).
Diagnostic momentum Once a diagnosis is made and repeated by others, other possibilities tend not to be considered.
Framing effect

How a story is told influences the likely diagnosis.

Visceral bias How we feel about the patient influences decision-making.
Anchoring bias Focusing on one particular piece of information or diagnosis and not considering further information of differentials. 
Premature closure A failure to consider other possibilities. It can be compounded by insufficient gathering of data from history and examination and/or the failure to appreciate relevance of risk factors.

Common systems errors

  • Excessive workload
  • Insufficient supervision
  • Poor communication and coordination of care
  • Lack of access (or not accessing) medical records
  • Test-related problems including unavailability, delays in conducting tests, delays or errors in reporting.

Debiasing strategies : Key strategies to mitigate cognitive bias

  • Do a thorough history and examination at the start, as approximately 80% of diagnoses can be made from this alone (BMJ Clin R p6). 

  • Learn more about evidence-based assessment to indicate or exclude a diagnosis.

  • During clinical reasoning, focus on firm facts as opposed to more vague data.

  • Routinely consider and document a short differential diagnosis, the most likely diagnosis (working diagnosis), accompanied by key points summarising the basis.

  • Routinely consider: 'What is important not to miss?'

  • Consider the worst case scenario and always seek information regarding must-not-miss diagnoses and red flags.

  • Keep in mind ‘what doesn’t fit’ i.e. don’t ignore the ‘odd’ symptom but rather consider what could be causing it. The level of certainty varies with the diagnosis e.g. leukaemia or UTI.

  • Engage the whole team in optimising care by articulating your clinical reasoning and asking ‘what else should I consider?' or ‘what am I missing?’ Typically, groups are cleverer than one smart individual.

  • If a patient’s clinical course does not occur as expected or they do not start improving, recheck the history and investigation results and review the differential diagnosis.

  • Ask for another relevant speciality to review the patient (a ‘fresh set of eyes’).

  • Use the Take 2 - Think, Do framework.

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