The document discusses the probabilistic nature of medical diagnosis and risk in clinical decision making. Some key points made include: 1. Physicians make probability assessments rather than definitive diagnoses and must accept some level of risk in their practice. 2. Even when a diagnosis is missed, harm does not always result as it depends on a sequence of subsequent events, and patients may improve on their own or be correctly diagnosed in the future. 3. Both sensitivity and specificity are important considerations for screening tests, but sensitivity is most important for ruling out a diagnosis while specificity prevents overdiagnosis of false positives. 4. There are risks and benefits to patients from diagnostic testing and treatment decisions, and physicians must weigh

1. Wrestling with risk

2. 4 Things
• You are not a diagnostician
• You are a risky doctor
• You get lucky (a lot)
• Diagnosis can be dangerous

3. You are not a
diagnostician

5. How sure are you?
• 98-100%
• 95-98%
• 90-95%
• 85-90%
• <85%

6. Cardiologists
Sens 79.9%
Spec 86.3%

7.  Sensitivity=a/a+c
 Specificity=d/d+b
Truth table and calculations

8. 705 patients in ED
129/705 had AMI
Sens 96.9%
Spec 54.7%

9.  Sensitivity 97%
 Specificity 54%
H-FAB in AMI
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10.  What does a +ve test
result mean?
 What does a -ve test result
mean?
What answers do we really
get from a Truth table?
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11. H-FAB testing for AMI
• A +ve test means they have
the diagnosis
• A +ve test means they
probably have the diagnosis
• A +ve test means they are
more likely to have the
diagnosis than before I did
the test
125125
44
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H-H-
FABPFABP

13. • Medicine is a science of
uncertainty and an art of
probability.

14. You are a
‘probablistician’

15. risky
doctors

16. Radiopaedia

20. What’s an acceptable
level of risk?

21. 1:50
1:25
1:10

22. Example - PERC rule
• Age>49
• Pulse>100
• Pulse Ox<95%
• Current haemoptysis
• Oestrogen use
• Previous venous thromboembolism
• Recent surgery
• Unilateral leg swelling
PERC Rule
sens = 97.4%
spec = 21.9%
^ Kline, JA; et al (2008).
"Prospective multicenter evaluation of the pulmonary embolism rule-out criteria".
Journal of Thrombosis and Haemostasis 6 (5): 772–780.

23. 1:50
1:25
1:10

24. Pre test probability for PE
6.9 % pre test probability of PE.

25. PERC Rule
sens = 97.4%
spec = 21.9%

26. PERC Rule
sens = 97.4%
spec = 21.9%
6.9 % pre test
probability of PE.

28. 21.9% negative

30. • Gestalt plus PERC rule will fail to spot two
PEs for every thousand patients
investigated.

31. • You get lucky (a lot)

37. Radiation Fear Bleeding

38. Did you miss the big one?

39. • are we happy???
• Gestalt plus PERC rule will fail to spot two
PEs for every thousand patients
investigated.

41. does a ‘miss’ matter?

42. QuickTime™ and a
decompressor
are needed to see this picture.
What does a miss really
mean?

44. Multi-step sequence
needed for harm to
take place

45. Diagnosis missedDiagnosis missed
They don’t comeThey don’t come
backback
It gets worseIt gets worse
The treatmentThe treatment
does not workdoes not work
Harm takes placeHarm takes place
They get betterThey get better
anywayanyway
They do comeThey do come
backback
The treatmentThe treatment
worksworks
Significant harmSignificant harm
avoidedavoided
Diagnosis madeDiagnosis made
..but the diagnosis is..but the diagnosis is
missed again!missed again!

46. • Probabalistic
• Risky
• Lucky
• Multi step sequence for harm

47. http://emcrit.org/misc/imaging-in-pe-diagram/
Just for fun

48. http://emcrit.org/misc/imaging-in-pe-diagram/
Just for fun

52. Is this all about PE?

53. • Good problem solving, sound judgement and
effective clinical decision making are
considered the highest attributes of physicians.
The theory and practice of clinical decision making
Croskerry P. Can J Anaesth 2005;52:1-8

54. Only when disclosure of uncertainty
becomes commonplace in medical practice
will the physician/patient relationship evolve
to a level of greater understanding and
satisfaction for both the physician and
patient.

55. 4 Things
• You are not a diagnostician
• You are a risky doctor
• You get lucky (a lot)
• The dangers of diagnosis

56. 4 Things
• You make probability assessments
• You take risks (but you understand them)
• Even when you miss, you usually get lucky
• Pursuing a diagnosis is not always right

57. • The only statistics
you can trust are the
ones you falsified
yourself.

62. Patient confidence
Physician certainty
Patient concern
Physician discomfort
Uncertainty, responsibility and the evolution of the physician/patient relationship
Henry MS JMedEthics 2006;32:321-323

64. I don’t have a PE
You don’t have a PE
I might have a small risk
of PE
That patient might have
a PE and sue me
Uncertainty, responsibility and the evolution of the physician/patient relationship
Henry MS JMedEthics 2006;32:321-323

65. To advance art and science in clinical
examination, the equipment a clinician most
needs to improve is himself.
Feinstein AR. Scientific methodology in clinical practice: IV.
Acquisition of clinical data. Ann Intern Med. 1964

67. Risk vs Benefit
• Benefits to patients
• Access to therapy
• Knowledge
• Treatment benefit
• Cost
• Risks to patients
• Denied therapy
• Risk of therapy
• Risk of investigation
• Cost

68. • Do I make diagnoses or probabilities?
• Is some risk acceptable in my practice?
• How do I communicate risk to patients?
Take away

71. What else do we need
to know?
• What’s the pre-test probability?
• What’s the performance of our tests?
• How many can we miss?

72. Tests
• Clinical risk scores
• D-dimer
• VQ
• Chest X-ray
• CTPA

73. Tests
• Clinical risk scores
• D-dimer
• VQ
• CTPA
Screening testsScreening tests
Definitive testsDefinitive tests

74. What’s an
acceptable
‘miss rate’ for
PE?
‘miss rate’ for
PE?

75. How many PEs are you
prepared to miss?
• 1 in 200
• 1 in 100
• 1 in 50
• 1 in 25
• 1 in 20
• 1 in 10

76. slide with flow chart of
diagnosis for PE
PatientPatient
ScreeningScreening
testtest
DefinitiveDefinitive
testtest
TherapyTherapy
Ruled outRuled out

78. PatientPatient
Diagnosis unlikelyDiagnosis unlikelyDiagnosis quite likelyDiagnosis quite likely
Patient treatedPatient treated Patient not treatedPatient not treated
Assessment/InvestigationAssessment/Investigation

79. PatientPatient
Diagnosis unlikelyDiagnosis unlikelyDiagnosis quite likelyDiagnosis quite likely
Patient treatedPatient treated Patient not treatedPatient not treated
Patient benefitsPatient benefits Patient has no benefitPatient has no benefit Patient is harmedPatient is harmed
Assessment/InvestigationAssessment/Investigation

80.  A
 B
 C
 D
A B C
Where do you set the cut
off?

83. Ruled out?

84. For a screening test
• Sensitivity is best for ruling out
• Specificity is best for ruling out
• Both are important

85. The danger of
overdiagnosis
• Risk of investigation
• Risk of treatment
• (especially for false positives)

86. Me

87. • 4 tests for Cardiac disease.
• One good rule in
• One good rule out
• 2 less than useful!
• Data given in 4 ways
• Sens/Spec
• NPV/PPV
• LR
• Graphical

89. Probability
• Diagnosis is a probability
• There are risks and benefits to patients by
embarking on a diagnostic pathway
• We will miss some people with disease
• We will mislabel (and treat) people without
disease

90. Which screening test?
• D-dimer rules out
• Wells score rules out
• PERC score rules out
• Wells and PERC rules out
• Wells + D-dimer rules out
• Other (e.g. dopplers, flip a coin, astrology etc)

91. Why diagnose?
 To label a condition
 To judge the severity of illness
 To predict subsequent course and prognosis
 To determine the response to treatment.
 Because it is useful to know

93. Text
c/o Tom Maisey
via Flickr

94. risky
doctors