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?
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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?
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a / (a + c)- What does specificity measure in a screening test?
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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?
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d / (b + d)- What does positive predictive value (PPV) tell you?
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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?
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a / (a + b)- What does negative predictive value (NPV) tell you?
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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?
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d / (c + d)- A test with very high sensitivity, when the result is negative, ____ the disease (SnNOUT).
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rules out- A test with very high specificity, when the result is positive, ____ the disease (SpPIN).
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rules in- How does increasing disease prevalence in a population affect a test's PPV?
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It increases PPV.- How does increasing disease prevalence in a population affect a test's NPV?
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It decreases NPV.- Do sensitivity and specificity change when disease prevalence changes?
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No, they are intrinsic properties of the test and remain constant regardless of prevalence.- What is the formula for the positive likelihood ratio (LR+)?
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Sensitivity / (1 - specificity)- What is the formula for the negative likelihood ratio (LR-)?
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(1 - sensitivity) / specificity
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