Updated: 12/31/2021

# Evaluating Diagnostic Tests

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• 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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(M2.OMB.18.1) Health officials are considering a change be made to the interpretation of the tuberculin skin test that will change the cut-off for a positive purified protein derivative (PPD) from 10 mm to 15 mm for healthcare workers. Which of the following can be expected as a result of this change?

QID: 210446
1

Decrease the sensitivity

39%

(11/28)

2

Decrease the specificity

14%

(4/28)

3

Increase the precision

4%

(1/28)

4

Increase the sensitivity

29%

(8/28)

5

No change to the sensitivity or specificity

7%

(2/28)

M 5 C

Select Answer to see Preferred Response

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(M2.OMB.18.1) A home drug screening test kit is currently being developed. The cut-off level is initially set at 4 mg/uL, which is associated with a sensitivity of 92% and a specificity of 97%. How might the sensitivity and specificity of the test change if the cut-off level is changed to 2 mg/uL?

QID: 210448
1

Sensitivity = 90%, specificity = 99%

25%

(3/12)

2

Sensitivity = 92%, specificity = 97%

17%

(2/12)

3

Sensitivity = 95%, specificity = 98%

8%

(1/12)

4

Sensitivity = 97%, specificity = 96%

50%

(6/12)

5

Sensitivity = 100%, specificity = 97%

0%

(0/12)

M 5 C

Select Answer to see Preferred Response

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