Applying biostatistics to clinical vignettes tested on USMLE Step 2 CK: interpreting screening test results, number needed to treat and harm, and likelihood ratios in patient scenarios.
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- What does sensitivity measure in a screening test?
- The proportion of people WITH the disease who correctly test positive (true positive rate).
- In a 2x2 table (a=true pos, b=false pos, c=false neg, d=true neg), what is the formula for sensitivity?
- a / (a + c)
- What does specificity measure in a screening test?
- The proportion of people WITHOUT the disease who correctly test negative (true negative rate).
- In a 2x2 table (a=true pos, b=false pos, c=false neg, d=true neg), what is the formula for specificity?
- d / (b + d)
- What does positive predictive value (PPV) tell you?
- Given a positive test result, the probability the person actually has the disease.
- In a 2x2 table (a=true pos, b=false pos, c=false neg, d=true neg), what is the formula for PPV?
- a / (a + b)
- What does negative predictive value (NPV) tell you?
- Given a negative test result, the probability the person actually does not have the disease.
- In a 2x2 table (a=true pos, b=false pos, c=false neg, d=true neg), what is the formula for NPV?
- d / (c + d)
- A test with very high sensitivity, when the result is negative, ____ the disease (SnNOUT).
- rules out
- A test with very high specificity, when the result is positive, ____ the disease (SpPIN).
- rules in
- How does increasing disease prevalence in a population affect a test's PPV?
- It increases PPV.
- How does increasing disease prevalence in a population affect a test's NPV?
- It decreases NPV.
- Do sensitivity and specificity change when disease prevalence changes?
- No, they are intrinsic properties of the test and remain constant regardless of prevalence.
- What is the formula for the positive likelihood ratio (LR+)?
- Sensitivity / (1 - specificity)
- What is the formula for the negative likelihood ratio (LR-)?
- (1 - sensitivity) / specificity