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Updated: Dec 31 2021

Evaluating Diagnostic Tests

  • Sensitivity, Specificity, PPV, NPV
    • These 4 measures describe how well diagnostic tests capture the true presence or absence of disease 
    • Sensitivity (SN)
      • % with disease who test positive
      • = a/(a+c) = TP/(TP+FN)
    • Specificity (SP)
      • % without disease who test negative
      • = d/(b+d) = TN/(FP+TN)
    • Positive predictive value (PPV) 
      • % positive test results that are true positives
      • = a/(a+b) = TP/(TP+FP) 
    • Negative predictive value (NPV) 
      • % negative test results that are true negatives
      • = d/(c+d) = TN/(FN+TN)
    • Cut-off point may be adjusted to optimize sensitivity and specificity, which are inversely related (cut-off point with decreased sensitivity is associated with increased specificity and vice-versa)
      • will also affect NPV and PPV 
        • i.e., decrease in sensitivity associated with decrease in NPV in the same population (due to higher false negative rates)
    • Sensitivity and specificity are intrinsic to the diagnostic test 
      • do not change with prevalence
      • PPV and NPV do change with prevalence
    • Receiver operating characteristic (ROC) curves are a graphical depiction of a test's performance
      • Y axis: sensitivity
      • X axis: 1-specificity
      • The higher the curve, the better the test
      • This is quantified by the AUC (area under the curve); an AUC of 0.5 states that the test performs no better than chance (bad test!), whereas an AUC of 0.9 suggests a better-performing test
  • Odds Ratio, Relative Risk, Attributable Risk
    • These measures describe the relationship between a risk factor and a disease
    • Odds Ratio (OR)
      • odds of having disease in expose group / odds of having disease in unexposed group
        • = ad/bc
    • Relative Risk (RR)
      • probability of getting disease in exposed group / probability of getting disease in unexposed group
        • = [a/(a+b)] / [c/(c+d)]
      • If RR = 1, there is no association between exposure and outcome 
    • Dose-repons'increased level of exposure shows an increased relative risk of developing/odds ratio of having a diseasecan be used in OR or RR to support causality
      • increased level of exposure shows an increased relative risk of developing/odds ratio of having a disease
      • can be used in OR or RR to support causality
    • Attributable Risk (AR)
      • risk in exposed group - risk in unexposed group
        • = a/(a+b) - c/(c+d)
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