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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
• 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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