Evaluating Diagnostic Tests - Stats

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-reponse
- 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)