DO Diagnostic Reasoning and Evidence-Based Medicine 1 — Questions and Answers
Question 1: Bayes' theorem in clinical medicine states that the post-test probability of disease depends on which two factors?
- Pre-test probability and test likelihood ratio (Correct answer)
- Sensitivity and specificity alone
- Patient age and symptom duration
- Sample size and confidence interval
Correct answer: Pre-test probability and test likelihood ratio
Post-test probability integrates the pre-test probability (prevalence/clinical suspicion) with the test's likelihood ratio to determine how much a result changes disease probability.
Question 2: When a test has high specificity, a positive result is most useful for:
- Ruling IN disease (SpPin) (Correct answer)
- Ruling OUT disease
- Determining treatment dosage
- Confirming prognosis
Correct answer: Ruling IN disease (SpPin)
A highly specific test has few false positives, so a positive result strongly indicates true disease and is used to rule IN the diagnosis (SpPin mnemonic).
Question 3: A 55-year-old male smoker presents with hemoptysis and a new lung mass on CXR. The pre-test probability for lung cancer is estimated at 60%. The CT-guided biopsy has a sensitivity of 95% and specificity of 99%. A positive biopsy result essentially:
- Confirms the diagnosis (very high post-test probability) (Correct answer)
- Still requires additional testing before treatment
- Is insufficient without PET scan correlation
- Reduces the probability of malignancy
Correct answer: Confirms the diagnosis (very high post-test probability)
With high pre-test probability and a highly specific test showing a positive result, the post-test probability of malignancy approaches near certainty.
Question 4: Which statistical measure describes the strength of association between an exposure and outcome in a case-control study?
- Odds ratio (OR) (Correct answer)
- Relative risk (RR)
- Hazard ratio (HR)
- Number needed to harm (NNH)
Correct answer: Odds ratio (OR)
Case-control studies calculate the odds ratio because they sample based on outcome (case/control), making it impossible to calculate true incidence rates needed for relative risk.
Question 5: Confirmation bias in clinical diagnosis refers to:
- Seeking information that confirms an initial diagnosis while ignoring contradictory evidence (Correct answer)
- Overestimating the prevalence of rare diseases
- Using recent memorable cases to guide diagnosis
- Anchoring on the first piece of diagnostic information
Correct answer: Seeking information that confirms an initial diagnosis while ignoring contradictory evidence
Confirmation bias leads physicians to favor data supporting their initial hypothesis and discount findings that suggest an alternative diagnosis.
Question 6: A 95% confidence interval (CI) for a relative risk that does NOT cross 1.0 indicates:
- The result is statistically significant at the p < 0.05 level (Correct answer)
- The result is clinically irrelevant
- The study had insufficient power
- The effect size is too small to matter
Correct answer: The result is statistically significant at the p < 0.05 level
When a 95% CI for RR excludes 1.0 (no effect), the finding is statistically significant at the corresponding p < 0.05 threshold.
DO Diagnostic Reasoning and Evidence-Based Medicine 1
Bayes' theorem in clinical medicine states that the post-test probability of disease depends on which two factors?