Diagnosing – Critical Activity
HINF 371 - Medical Methodologies
Session 7
Objective
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To review the process of diagnosing
and factors undermining effective
diangosis
Reading
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Mar CD, Doust J, Glasziou (2006) Chapter 4: Diagnosis, in
Clinical Thinking: Evidence, Communication and DecisionMaking, Blackwell Publishing and BMJ Books, USA
Groopman, J (2007) Flesh-and-Blood Decision Making, in How
Doctors Think, Hougton and Mifflin, New York, USA
Critical Step
Being able to diagnose is a critical
skill in medicine
 Further investigation, treatment and
prognosis depends on the
effectiveness of this step

Effectiveness of Diagnosing
Depends on;
Listen patients
 Know which questions to ask
depending on the patients
 Determine the presence or absence of
the relevant clinical signs
 Know how to identify these signs
 Know which tests, if any, to order

Diagnosing is

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Putting the patient in a category that we
believe they are similar
However,

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Categories change
New categories are developed
ICD – 10
These change because

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Prevalence of diseases in population change
Increased ability to identify (test)
Diagnosis is important because
Gives options regarding treatment
and management of patients
 Predict prognosis and response to
treatment
 Allows patients to categorize
themselves
 Allows patients to manage their role
in society and their emotions

Three types of diagnosing
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Pattern recognition and feature matching:
situation is familiar and diagnostic
information available
Hypothetico-deductive reasoning: more
challenging problem, 3 to 5 diagnosis at a
time, collect information to confirm or
exclude hypothesis
Information gathering: not enough
information to generate a hypothesis,
unfamiliar or non-specific disease
Diagnostic Errors


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Increased quantity of information (rather
than quality) is related to increase rate of
inaccurate diagnosis
Expert physicians collect less data but more
discriminating on what to collect and they
are able to generate accurate hypothesis
earlier
Over estimation of the information gained
from laboratory or imaging
Ordering test ‘just to be certain’ – VOMIT
syndrome (victims of modern imaging
technology)
Diagnostic errors


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Missed diagnosis (false negative)
Wrong diagnosis (false positive)
Causes

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Disease knowledge base: recognize symptoms
and link them to diagnosis
Communication skills
Examination skills
Diagnostic reasoning skills: assess baseline
probability, interpret the findings, use of
diagnostic heuristics, ability to recognize new
symptoms to revise diagnosis
Organization skills: follow-up; peer-to-peer
communication
Faults in Diagnostic reasoning
1.
2.
3.
Faulty triggering
Faulty context information
Faulty information gathering and
processing
1.
2.
3.
4.
5.
Errors in assessment of disease prevalence
Errors in interpretation of clinical data:
sensitivity and specificity, no weight of
symptoms
Errors in application of axioms
Faulty verification
‘No fault’ errors
In conclusion
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Both doctors and patients must accept that
a certain degree of uncertainty will always
be present in medical diagnosis
More effort required for diagnostic
reasoning teaching
Increased use of probabilistic reasoning
Use of red flag system
Time course of different illnesses – vascular
quick development, infection inflammation
in days, cancer over weeks to months
Action thresholds

Diagnosis

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Positive likelihood ratio (sensitivity/(1specificity) >2 helpful in ruling in, >10 very
good for ruling in
Negative likelihood ratio ((1specificity)/Specificity) <0.5 helpful in ruling
out, <0.1 very good for ruling out
Test ordering

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Probability of disease <0.07 – no treatment
Probability of disease >0.2 – test then treat
Probability of disease >0.65 – treat