ST1a teaching 2016
watch this !
 https://www.youtube.com/watch?v=xuZl9tRqjoQ
Diagnosis
What diagnosis do you think of?
 Discuss the differential diagnosis for each case.
A 19 year old woman presents with lethargy, rash,
jaundice and a palpable liver. She was given
amoxicillin for a sore throat 5 days ago.
A 21 year old woman presents with lethargy and
jaundice. She has recently travelled to the
Middle East and engaged in casual sex there.
Occam’s razor
 Both these cases represent the philosophy of William
of Occam – a Franciscan friar who stated that “entities
should not be multiplied beyond necessity”
 i.e. diagnostic parsimony
 The clinician should seek a single diagnosis rather
than diagnosing two or more unrelated ones.
 This is the first diagnostic pearl of wisdom.
Another case
 A 50 year-old woman presents with weight loss,
lethargy, thirst, and polyuria. On examination, she
has a tachycardia of 110 bpm regular, looks thin, and
has a tremor.
This patient had both new thyrotoxicosis and new
diabetes mellitus (presenting more dramatically
because of the thyrotoxicosis). Relates to the next
diagnostic pearl of wisdom.
Hickam’s dictum
 John Hickam graduated from Harvard in 1940 and
stated that multiple symptoms and signs may be due
to more than one disease.
 Or “patients can have as many diseases as they damn
well please”
 This may also have statistical validity - it is more likely
that a patient has several common diseases rather than
one rare disease that explains multiple symptoms.
 for older patients, the scales may tip in favour of
Hickam.
How do doctors diagnose.
 Medical students and hospital doctors still use the
inductive (classical) method based on a
comprehensive history (HPC), full systematic enquiry,
PMH and DH, FH and SH and physical examination.
 A diagnosis is sought out ‘at all costs’.
 This method is systematic, indiscriminative and
expensive in time and resources.
 Furthermore the “all you can scan” body investogram
and the battery of bloods provides a rat’s maze of blind
alleys to pursue.
What diagnosis comes to mind?
 A 47 yr old man consults because he has had diarrhoea
for the past 10 days with occasional cramping
abdominal pain and feeling off colour.
 3 weeks ago he travelled to Bangkok for an academic
convention.
 He has not lost weight or noticed blood or mucus in
his stools.
 He has taken loperamide for the past few days without
much benefit.
The hypothetico-deductive model
 In GP, we approach consultations differently.
 Doctors begin to formulate diagnostic possibilities
very early on in the consultation – these hypotheses
are based on cues (verbal and non-verbal), obtained
from the patient and from their medical record.
 As the consultation develops, these hypotheses are
ranked and further information is gathered (by history
and exam) to confirm or refute them.
 New ideas may develop and hypotheses may be revised
if the above reveals no support.
Iterative diagnosis
 Sometimes called the iterative model meaning
recurrently testing the hypotheses.
 Clinicians use many such shortcuts (heuristics) in
clinical reasoning – this is not a fault – they are
typically correct and allow them to arrive at a working
diagnosis with the minimum of delay.
 This can avoid excessive testing and anxiety.
 Exhaustive data collection without hypotheses usually
does not improve diagnostic accuracy and may make it
worse.
When is iterative diagnosis used
 GPs will recognise that they formulate one or more





presumptive hypothesis when the patients starts to
speak
The acute onset of vertigo when rolling over in bed
The GP listens to history through the filter of the
initial hypothesis
Does the vertigo come on with head movements and
become less severe over a minute or two ?
The GP’s examination will be directed to supporting or
refuting a hypothesis
Is the Hallpike test positive ?
What are you thinking?
 A women presents, with headaches, and this time with
her husband who is concerned.
 The headaches are getting worse and she seems more
“vague” at times.
 She was seen 4 days ago with sudden onset of severe
headache developing over about 3-4 hours. There was
no visual disturbance and neurological examination
was entirely normal.
 She was given dihydrocodeine to take with otc
paracetamol.
What questions are you going to ask?
Iterative diagnosis
 It is an essential component of medical exercise
 Involves rapid, simultaneous generating and testing of
hypotheses. It is usually fast, efficient and accurate.
 A GP will formulate one or more presumptive
hypotheses as the patient walks into the room and
starts talking – this will direct the history.
 Examination will usually be directed towards
supporting or refuting a hypothesis.
 Sometimes intuition will apply a brake to the
analytical reasoning –’something doesn’t fit’
Stages in reaching a diagnosis
 Iterative diagnosis can otherwise be described as
diagnostic reasoning




Can be split into a 3 stage model:
Initiation of diagnostic hypotheses
Refinement of the diagnostic hypotheses
Defining the final diagnosis
 Not all stages are necessary. For instance a spot
diagnosis of acne at the initiation stage may miss out
the refinement stage.
Initiation stage
 Spot diagnosis is almost instantaneous and used in
about 20% of cases. Often skin diagnoses such as
eczema or moles.
 Self diagnosis is expressed by about 20% of patients –
can be accurate in recurrent UTIs in women or often
‘mislabelled’ such as “I have a chest infection”.
 Presenting complaint such as “ I have a headache” is
used most often by GPs – over 60% of cases.
 Pattern recognition trigger is least common such as
thirst, weight loss and feeling unwell in a teenager.
Refinement stage
 Restricted rule outs – knowing the commonest cause of the
presenting problem and ruling out serious diagnosis ( headache, TTH,
SAH and TA).
 Stepwise refinement – based on anatomy or pathology.
Such as wrist pain is located to radial side and linked to the thumb.
 Probabilistic reasoning – specific (but imperfect) use of symptoms,
signs or diagnostic tests to rule in or rule out a diagnosis. Such as urine
dipstick in UTI.
 Pattern recognition fit – sx and signs are compared to previous
patterns/ cases and a disease is recognised when the pattern fits.
commonly used. E.g. acute MI
 Clinical prediction rule – is a formal version of pattern recognition –
such as ABCD for stroke risk or Ottawa ankle rules.
Final definition stage
 Known diagnosis – in nearly 50% cases, a sufficient level of certainty




is present to start appropriate treatment . Such as viral URTI or warts.
Ordering further tests – a standard test can rule in/ out the disease,
such as a MSU for UTI. Or a colonoscopy for red flags of bowel cancer.
Test of treatment – when the diagnosis is uncertain, the response to
treatment is often used to refute or confirm it. Such as inhalers for
nocturnal cough.
Test of time. A “wait and see” strategy allows the diagnosis to become
more obvious such as a patient with abdo pain, D&V with no red flags
who is diagnosed as having viral gastroenteritis when better after 1-2
weeks.
No label applied – where no diagnostic lable could be assigtned to tha
patient, presentations were often vague and did not fit a recognisable
pattern.
Coffee break
 Welcome back
 Now – watch this.
 https://www.youtube.com/watch?v=_9sAUNhGPk0
 Now we are going to discuss how diagnoses may be
wrong and how errors can occur
How does diagnosis go wrong
 In small groups of 3-4, try to think of a couple of cases
where a diagnosis was missed or delayed.
 What factors played a part in this
 What key decisions were made that resulted in an
adverse outcome.
 What occurred to enable the diagnosis to be made.
How does diagnosis go wrong ?
 When errors in diagnosis occur, they are rarely due to a
lack of knowledge or carelessness.
 No doubt, some errors are as a consequence of poor
data gathering
 But critical data is often missed simply because the
clinician was not thinking of the correct diagnosis.
 Although clinicians gather less data as they gain
experience, this does not seem to have a negative effect
on diagnostic accuracy.
Cognitive errors
 When diagnoses are missed, they are often due to one
or more of a set of predictable cognitive errors.
 An example of how a diagnosis can go wrong is
attributing a set of symptoms to an ‘easier’ diagnosis.
 Such as retrosternal chest pain with burping to GORD
because it is worse after a fatty meal. But the patient is
male, in 50s and h/o smoking.
 Wrong diagnoses can be caused by atypical
presentations or very rare conditions.
Confirmation bias
 Confirmation bias – gathering information that will
confirm rather than refute the diagnosis.
 Confirmation bias is a tendency for people to favor
information that confirms their preconceptions or
hypotheses, regardless of whether the information is
true
 As a result, people gather evidence and recall
information from memory selectively, and interpret it
in a biased way
 Don’t ignore that which doesn’t fit
Confirmation bias (2)
 Karl Popper (Austrian philosopher) stated that “if you




suppose all swans are white, you will not find any
swans but white swans”
One should, in fact, be searching for black swans
Called the “falsification principle”
So formulate testable hypotheses – remind ourselves
to consider alternatives to our initial diagnosis.
“What can I not afford to miss ?”
One example is a headache - “like a migraine with visual
aura” in a 70 year old lady !
Base rate neglect
 Forgetting that common diseases are common.
 “when you hear the sound of hooves think of horses
not zebra”
 This can be linked to inappropriate confirmation selective use of a minor abnormality to confirm an
incorrect diagnosis.
This could be linking a mildly raised TSH as a cause of
fatigue when depression is more likely and common.
Availability bias
 Relating the case to an easily recalled example.
 This can sometimes be due to anchoring – sticking to
an initial diagnosis despite disconfirming evidence.
 This can be avoided by specifically going after signs
and symptoms that would be inconsistent with the
diagnosis, thus suggesting alternatives.
The patient presenting with vertigo is specifically asked
and tested for facial weakness.
Availability bias (2)
 A 44 yr old gentleman with chronic cough is
diagnosed with GORD after attending a recent ENT
talk explaining that this is common and often treated
inadequately.
 The patient (non-smoker) has a long h/o heartburn
often treated with otc antacids so you start a PPI and
review in 3 months. (chest is clear)
 At review, you are away and the next doctor notes that
the cough is dry and worse at night and in cold
weather. He works as a paint-sprayer.
Premature closure
 Arriving at a conclusion before gathering the critical
data and not revisiting it.
 This is the dominant cause of diagnostic errors
 This could be avoided by reconsidering dissonant facts
– re-examine facts that don’t quite fit.
An example is diagnosing intermittent claudication in
a non-smoker with exertional leg pains when further
questioning would have resulted in a diagnosis of
lumbar stenosis.
Premature closure (2)
 MPS case (2016)– 28 week pregnant lady (primip) asks for





HV – acutely unwell. Short h/o nausea and headache, as
well as swollen ankles.
The antenatal record showed weight gain of 25kg and the
BP was ‘mildly elevated’.
No record of urinalysis
The patient was prescribed gaviscon and a diuretic and told
to rest.
Later that day, she had loss of vision followed by a grand
mal seizure. She went to hospital by ambulance for emerg
CS and baby was born with CP.
Diagnosis was pre-eclampsia. Expert opinion was critical of
no urinalysis or referral to hospital.
Premature closure (3)
 MPS case (2015) 60 yr old HGV driver – overweight and
smoker – walking limited by calf pain. Increasing pain right
foot/ calf – presented to ED and DVT excluded. Advised to
see GP for “circulation problems”.
 Increasing pain at rest for next 2 nights – foot felt cold and
numb. Had to get up and walk around to relieve the pain.
The GP the following day noted a cool, pale right foot and
organised non-urgent Doppler assessment. And prescribed
quinine.
 He then attended ED a few days later, following advice
from GPP over the phone, who noted weak pulses and
diagnosed arterial insufficiency. Advised to attend doppler
next week as planned.
Premature closure 3 (cont)
 The patient attended Doppler exam the following
week with the operator noting an acute painful,
swollen calf with no result available due to pain.
 The patient was seen the next day by the same GP,
referred to SDU the following day and admitted from
there. Angiogram showed SFA thrombus.
 Embolectomy unsuccessful so proceeded to bypass but
leg was not viable resulting in AKA. Resulting in
phantom limb pain and loss of daily activities.
 MPS could not defend the claim due to lack of
diagnosis of acute critical ischaemia and lack of
urgency of referral.
Framing bias
 This is being swayed by the way in which the problem
is phrased.
 A patient with headache is asked a direct question
about nausea with the reply “a bit” then being
recorded as ‘vomiting consistent with migraine’.
 This can lead to a failure to gather adequate data or
cognitive oversight – simply not thinking of the
correct diagnosis.
 A patient will sometimes be reluctant to provide
information if it suggests a serious diagnosis.
Bayes’ theorem
 Thomas Bayes (1701-61) was an English Presbyterian
minister
 In Bayesian probability, one starts with an estimate of
probability
 one refines that probability based on subsequent
observations to create a new updated probability
 Clinicians use their experience, the clinical features
and results of diagnostic investigations to refine the
likelihood of each cause and then choose the most
likely.
Is Bayes the wisest of them all?
 to use Bayes effectively, adequate experience is
required
 If we are inexperienced or find ourselves in an
unfamiliar situation then we do not have the previous
observations required to help refine the initial
probability
 So remain open minded and be prepared to
challenge one’s diagnosis
 Knowing when to continue testing and when to stop
and accept the current theory is the diagnostic art of
the clinician
How can we improve
 Awareness of common pitfalls helps us to avoid them
 Specific and simple strategies can lead to real
improvement
 Common denominator in these strategies is reexamining the data and reconsider the formulation
(iterative diagnosis)
Six strategies
 Routinely second guess – routinely consider
alternatives to initial diagnosis. What can I afford not
to miss. Is a red painful foot gout or critical ischaemia?
 Seek data that does not fit with the hypothesis –
specifically go after signs and symptoms that would be
inconsistent or suggest alternatives. Asking about jaw
claudication in a person with headache.
 Reframe when recording – consciously re-examine
the hx as we write the notes. Did we ask direct closed
questions that may be misleading.
Six strategies (2)
 Reconsider dissonant facts – facts do not quite fit.
The migraine patient has her worst headache after.
 Use time as a diagnostic test – allows the GP to
review the diagnosis and separate minor and time
limited conditions from more serious problems. A
patient with a cough could be expected to get better
within 3 weeks.
 Know about test accuracy – this is awareness of preand post-test probabilities. And also accuracy – a
raised serum urate does not mean that painful joint is
gout.
Diagnosing headache in primary care - BJGP
study (2014) – qualitative study
 4 broad themes:
 Knowing the patient
 First impression
 Intuition/ experience
 Application of the test of time.
 ‘established guidelines did not play a role’
Finally:
 Next topic is managing uncertainty
 Try to think of cases where there has been diagnostic
uncertainty and how you dealt with this.
 Remember to hand in feedback forms please.