Key Concepts Taught

Teaching Diagnostic Reasoning

Goutham Rao, MD

Associate Professor, University of Pittsburgh

Key Concepts Taught

Introduction to Medical Decision Making

Mandatory course for first year medical students

Replaced “Clinical epidemiology and biostatistics” in 2001.

Key Concepts Taught

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.

Key Concepts Taught

Key Concepts Taught

Historical perspective (e.g. diagnosis in ancient times).

Differentiating diagnostic strategies InductionDeductionPattern recognition

Instance basedPrototype based

Key Concepts Taught

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?

Key Concepts Taught

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?

Key Concepts Taught

Four Simple Questions

You know your patient’s 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 doesn’t have it?” = specificity

You know your patient’s 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 doesn’t have the disease?” = NPV

Key Concepts Taught

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 byThe proportion who have the same test

result among those without the diseaseLR = TEST RESULT/DISEASE + TEST RESULT/DISEASE –

Key Concepts Taught

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)

Key Concepts Taught

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.

Key Concepts Taught

Influence of Pre-Test Probability

Assume that the test is positive in all patients:

Pre-test probability

Pre-test odds

Post-test odds

Post-test probability

Mr. D

75% 3/1 15/1 0.94

Ms. A

1% 1/99 5/99 .048

Mr. Y

25% 1/3 1.67/1 .62

Key Concepts Taught

Important Principle

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

Key Concepts Taught

Errors in Estimation of Pre-Test Probabilities

AvailabilityRepresentativeness

Key Concepts Taught

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 __

Key Concepts Taught

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.

Key Concepts Taught

ROC Curves