Teaching Diagnostic Reasoning

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Teaching Diagnostic Reasoning

• Goutham Rao, MD
• Associate Professor, University of Pittsburgh

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Introduction to Medical Decision Making

• Mandatory course for first year medical students
• Replaced Clinical epidemiology and
  biostatistics in 2001.

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Core Principles

• Traditional journal clubs overemphasize critical
  appraisal.
• Understanding the quantitative aspects of
  research design and data analysis promotes an in
  depth understanding of original research.
• As they read a research article, instead of
  asking themselves whether the article is useful
  or not, students ought to ask, What is useful
  about this article?
• Physician numeracy can form the basis of an EBM
  course.
• The content of an EBM course should be based upon
  what numeracy concepts and skills students
  require to accurately interpret evidence and
  apply it to individual patients.

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Key Concepts Taught

• Historical perspective (e.g. diagnosis in ancient
  times).
• Differentiating diagnostic strategies
• Induction
• Deduction
• Pattern recognition
• Instance based
• Prototype based

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Deductive and Inductive Reasoning

• Arthur is a 24-year-old third year medical
  student beginning his first clinical rotation on
  an internal medicine ward in a large teaching
  hospital. One morning he is given two tasks by
  his supervising physician. For his first task, he
  is told, Arthur, I think the patient in room 330
  has rheumatic fever. Go find out if he has it or
  not. For his second task Arthur is told, The
  patient in room 405 is feeling unwell and passed
  out before coming to the hospital this morning.
  Let me know what you believe she may have.
  Arthur has very little clinical experience. How
  do you think he will go about each task? Which
  task will prove more difficult for him?

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Test Characteristics

• Scenario

Your friend Jack has just invented a new test for
colon cancer that involves just taking a small
blood sample and combining it with a reagent that
reacts to cancer cells. Jack developed his test
after years of studying the blood of patients
with and without colon cancer and identifying
certain markers in the blood of patients with
cancer that could be identified with a reagent.
He asks you to determine the quality of his new
diagnostic test. How will you go about this task?
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Four Simple Questions

• You know your patients disease status
• 1. I know my patient has the disease. What is
  the chance that the test will show that my
  patient has it? sensitivity
• 2. I know my patient does not have the disease.
  What is the chance that the test will show that
  my patient doesnt have it? specificity
• You know your patients test status
• 3. I just got a positive test result back on my
  patient. What is the chance that my patient
  actually has the disease? PPV
• 4. I just got a negative test result back on my
  patient. What is the chance that my patient
  actually doesnt have the disease? NPV

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The Likelihood Ratio

• Ratio of two proportions
• The proportion who have a particular test result
  (e.g. positive, negative, high probability) among
  those with a disease
• divided by
• The proportion who have the same test result
  among those without the disease
• LR TEST RESULT/DISEASE
• TEST RESULT/DISEASE

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Thomas Bayes (1702 1761)

• English mathematician and Minister
• Essay towards solving a problem in the doctrine
  of chances
• Method for evaluating new information in
  conjunction with prior information.
• Example (Economist)

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Three Patients

• Patient One Mr. D. is an obese sixty-year old
  smoker with a long-standing history of diabetes,
  high blood pressure and high cholesterol who
  complains of two hours of crushing retrosternal
  chest pain.
• Patient Two Ms. A is a twenty-one year old
  college student who complains of episodic
  twinges in the left side of her chest for the
  past two days. She does not smoke, take any
  medications, or have any medical history of
  significance. She admits to being under a great
  deal of stress recently, as final exams are about
  to begin.
• Patient Three Mr. Y. is a 47-year old smoker
  who complains of an unusual pressure in his
  chest for the past five hours. He has not had
  symptoms of this type before. He has no history
  of hypertension, diabetes, dyslipidemia or family
  history of heart disease. He exercises regularly
  without any chest discomfort.

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Influence of Pre-Test Probability

• Assume that the test is positive in all patients

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Important Principle

• Diagnostic tests are most useful in patients with
  intermediate pre-test probabilities.

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Errors in Estimation of Pre-Test Probabilities

• Availability
• Representativeness

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Conjunction Fallacy

• 134 beginning medical students completed the
  following problem
• Amelia is a twenty-three-year-old medical student
  who comes to your office for help. You suspect
  she has the common cold. In the blank spaces
  below, based on your knowledge and experience
  with the common cold, estimate the probability
  that Amelia would experience each of the
  following symptoms or symptom combinations. For
  example, if you believe Amelia has a 100 chance
  of experiencing b and a 90 chance of
  experiencing c put 100 and 90 in the
  respective blanks.
• runny nose and diarrhea __
• fatigue
  __
• diarrhea
  __
• ear pain and shortness of breath
  __
• sore throat
  __
• headache
  __

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Diagnostic Research Phase II example

• A new questionnaire test has been developed to
  determine if a patient is suffering from a
  migraine headache. The patient simply answers the
  questions shown below. One point is assigned to
  each yes.
• Questionnaire administered to 200 sufferers of
  frequent headaches.
• All patients have gold standard as well.

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ROC Curves