Clinical Decision Making How Doctors Think

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Clinical Decision Making- How Doctors Think

• Professor Frank Dobbs
• Institute of Postgraduate Medicine and Primary
  Care

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Clinical Diagnosis

• Principles
• Specialists and GPs
• How Librarians can help
• Streptococcal Throat Infection

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Principles of Diagnosis

• Levels of Diagnosis
• The Success of Science
• Recording Diagnoses
• Diagnostic Methods

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Levels of Diagnosis

• Campbell 1977
• Syndrome
• Collection of symptoms and signs
• Structural Change
• Damaged organs and tissues
• Cause
• Underlying defect or infection

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Levels of Diagnosis

• Patients Belief about Illness
• Wellbeing
• Syndrome
• Collection of symptoms and signs
• Structural Change
• Damaged organs and tissues
• Cause
• Underlying defect or infection

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Levels of Diagnosis - Treatment

• Patients Belief about Illness
• Restore Health
• Syndrome
• Palliation
• Structural Change
• Encourage Healing
• Cause
• Cure

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The Success of Science

• Logical Belief System
• Constant Syndrome
• Reversible Changes
• Single Cause

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Medical Records

• Weed 1969
• Subjective Information
• Objective Information
• Analysis
• Plan

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Medical Records

• Subjective Information
• Objective Information
• Patients Beliefs/Worries
• Analysis
• Plan

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Diagnostic Method

• Hutchisons Clinical Methods. Bomford et al 1975
• "There are two steps in the making of a
  diagnosis. The first is observation ....the
  second is interpretation of the information
  obtained."
• Obtain Data
• Interpret Data
• Repeat

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Diagnostic Strategies

• Macartney 1987
• Blunderbuss Approach
• Algorithm
• Hypothesis Testing
• Probability Diagnosis

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Blunderbuss Approach

• Collect all available data
• History
• Examination
• Tests
• Look for patterns
• Wasteful
• Time, Resources
• General Practice (Cradle to Grave records)

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Algorithm

• Series of Questions
• Protocols
• NHS Direct, Medical Assistants, Minor Injury
  Units
• Time Consuming
• Uncertainty
• Incorrect data, Missing data

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Hypothesis Testing

• Generate Hypothesis
• Logical Probabilities (age, sex, common
  conditions)
• Pattern Recognition Intuitive. Imaginative
• Scientific Approach
• Deduce from underlying laws of nature
• Paradigm shift
• Test hypothesis

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Hypothesis Testing

• Elstein et al 1972
• Four Hypotheses (/- 1)
• Reject
• Include
• Re-consider
• Cost
• Wasted testing of rejected hypotheses
• Level of certainty

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Probability Diagnosis

• Ranked Hypotheses
• Differential Diagnosis List
• New Data Item
• Alters rank
• Combine Probability
• Bayes Theorem
• Choose Data Items which Differentiate Best
• Efficient, Rapid

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How Probability Diagnosis is Used

• Intuitive Use
• Specialists
• GPs
• Calculating Probability
• Bayes Theorem
• B-scores
• How Libraries Can Help
• Streptococcal Throat Infection

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Intuitive Use

• Prior Probability
• Main difference between Specialist and GP
• Selected population
• Listen to the Patient
• Four Most Likely Diagnoses
• Hypotheses/Pattern recognition
• Best Indicators for Illnesses
• Confirmation and exclusion
• Tests may be Best Indicators

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Bayes Theorem

• Rev. Thomas Bayes, 1763 (reprinted 1973)
• How to predict a certain condition
• Independent data items
• Updating the probability due to each item
• Likelihood Ratio
• Posterior Probability

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The Formula

• Prior Probability
• Likelihood Ratio
• Posterior Probability
• Post.ProbabilityPrior Probability x Likelihood
  Ratio

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Likelihood Ratio

• Group of Patients with Bacterial Vaginosis
• Group of Patients without BV
• Data item (Whiff test ve)

No BV (8)
BV (56)
Pos.Likelihood Ratio 56/8 7
Neg.Likelihood Ratio 44/92 0.48
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B-Scores

• Logarithm of Likelihood Ratio
• Add scores
• Multiplied by a constant to produce Single Digits
• i.e. 1-9 (clinical conditions)
• Easy to add in your head
• Score for Presence or Absence of each Item

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Using B-Scores

• Add Score for Presence or Absence of Each Data
  Item
• Add Score for Prior Probability (Incidence)
• Rule of Thumb to Interpret Total Score
• 2 2 to 1 odds in favour of condition
  being present
• Odds double for 2 increase
• 4 4 to 1
• 6 8 to 1
• Negative scores
• -2 2 to 1 against
• Zero 50/50 chance

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B-Scores for BV

• 
  Present Absent
• Smell (Vaginal Discharge) 2
  -3
• BV in Last 12 Months 4
  -2
• White Discharge -1
  3
• Frothy Secretions 4
  -1
• pH gt 4.5 2
  -4
• Whiff Test 5
  -2
• Prior Probability
  1

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Performance

• All 13 Significant Factors
• Sensitivity 82
• Specificity 73
• 6 Best Factors
• Sensitivity 78
• Specificity 74
• Common Clinical Practice
• Sensitivity and Specificity around 65

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Recommended Use

• Use scores regularly for 2 months
• Always examine
• Send off Swabs for Gram Stain (BV)
• BV Incidence (55 - 62 1 Prior Probability
  Score)
• Audit of diagnosis
• Self-collected swab for pH and Whiff test
• Score if pH and Whiff test disagree
• Examine discharge if score between -7 and 2

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How Libraries Can Help

• Sensitivity and Specificity
• Likelihood Ratios
• Not Odds Ratios
• STARD Standards for Reporting of Diagnostic
  Accuracy (Bossuyt et al 2003 BMJ)
• Systematic Reviews (Deeks 2001 BMJ)

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Streptococcal Sore Throat

• Development and implementation of a scoring
  system to reduce inappropriate antibiotic
  prescribing for sore throat
• Dobbs F, Jordan A, Fleming D

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Viral throat

• 70 of sore throats
• Soluble Aspirin or Paracetamol
• Plenty of drinks

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Streptococcal throat

• 30 of sore throats
• Antibiotic - one day shorter
• Symptomatic treatment as before

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Strep Sore Throat B-scores
If the total is more negative than -2 then
an antibiotic will probably not help
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GP Enrolment

• 14 GPs
• Random allocation
• One pair each month
• 12 months

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Groups

• Usual Practice group
• B-score group
• Diagnosis v. Throat swab result
• Outcomes
• Correct treatment decision
• Antibiotic use

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Patient Enrolment

• 170 patients enrolled
• B-score group 82
• Usual Practice group 88

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Diagnostic Performance

• Sensitivity 71
• Specificity 71

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Correct Treatment

• B-score 69
• Usual Practice 48
• (i.e. B-score 44 better)

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Antibiotic Use

• B-score 47
• Usual Practice 65
• (i.e. 38 more antibiotics in Usual Practice
  group)

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Implementation

• Health Authority GP Education
• Cornwall IoS
• Birmingham
• Nottingham
• South West Devon
• GP Formulary
• Irish College of General Practitioners
• Sore Throat Guideline

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Implementation

• Health Authority GP Education
• Cornwall IoS
• Birmingham
• Nottingham
• South West Devon
• GP Formulary
• Irish College of General Practitioners
• Sore Throat Guideline