Updated: 11/28/2020

Evaluating Diagnostic Tests

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Sensitivity, Specificity, PPV, NPV
  • 2x2 Table Sensitivity and SpecificityThese 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
  • 2x2 TableThese 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)
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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? Tested Concept

QID: 210446
1

Decrease the sensitivity

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(1/4)

2

Decrease the specificity

25%

(1/4)

3

Increase the precision

0%

(0/4)

4

Increase the sensitivity

25%

(1/4)

5

No change to the sensitivity or specificity

25%

(1/4)

M 5 B

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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? Tested Concept

QID: 210448
1

Sensitivity = 90%, specificity = 99%

0%

(0/2)

2

Sensitivity = 92%, specificity = 97%

50%

(1/2)

3

Sensitivity = 95%, specificity = 98%

50%

(1/2)

4

Sensitivity = 97%, specificity = 96%

0%

(0/2)

5

Sensitivity = 100%, specificity = 97%

0%

(0/2)

M 5 B

Select Answer to see Preferred Response

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