Understanding Sensitivity & Specificity in Diagnostic Tests

1. appreciate • appraise • apply Part 3 Brenda Boucher PT, PhD, CHT, OCS, FAAOMPT

2. x positive liklihood ratio sensitivity negative liklihood ratio specificity “Ultimately, we would all like to deal with certainty,(but) the diagnostic process is one of probability.”3

3. EBP • Understand the purpose of tests • Determine accuracy of tests

4. Statistical Measures

5. Sensitivity & Specificity Sensitivity & Specificity are statistical parameters that describe the measurement validity of a test with regard to the relationship between the findings on the test compared to the findings on the reference standard. The usefulness of tests with high sensitivity or specificity pertains to their ability to recognize the presence of absence of a condition.

6. Sensitivity & Specificity “Given test results does my patient have the condition?”

7. Sensitivity & Specificity “Given the presence or absence of a condition, does this test give me the correct results?” Clinicians look at sensitivity and specificity values when selecting tests that will provide the most accurate result when predicting the presence or absence of a condition

8. Sensitivity & Specificity Drop Arm Test: Sensitivity = 8% Hawkins Test: Sensitivity = 92%

9. 2 X 2 Contingency Table Reference Standard Condition Present Condition Absent True False ✔✔✔ ✔✔✔ ✔✔✔ ✔✔✔ Test Results

10. 2 X 2 Contingency Table Reference Standard Condition Present Condition Absent True False ✔✔✔ ✔✔✔ Positive Test Results ✔✔✔ ✔✔✔ Negative

11. 2 X 2 Contingency Table Reference Standard Condition Present Condition Absent True False + True Positive + False Positive Positive Test Results − False Negative − True Negative Negative

12. Sensitivity Reference Standard Condition Present Condition Absent True False + True Positive + False Positive Positive Test Results − False Negative − True Negative Negative Sensitivity = True Positives / True Positives + False Negatives

13. Reference Standard Condition Absent Condition Present + True Positive + False Positive Positive Test Results True False − True Negative - False Negative Negative Sensitivity = True Positives / True Positives + False Negatives + + + Sensitivity - + + - + + + + + Condition Present

14. Reference Standard Number of subjects: 24 Condition Absent Condition Present + True Positive 10 + False Positive Positive Test Results True False − True Negative - False Negative 2 Negative Sensitivity = 10/10 + 2 = 83% + + + - + + - + + + + + Condition Present Condition Absent

15. Reference Standard Condition Absent Condition Presnt 10 True Positive 12 False Positive Positive Test Results True False 0 True Negative 2 False Negative Negative Sensitivity = 10/10 + 2 = 83% + + + + + + - + + + + - + + + + + + + + + + + + Condition Present Condition Absent

16. Reference Standard Condition Absent Condition Present 10 True Positive 0 False Positive Positive Test Results True False 12 True Negative 2 False Negative Negative Sensitivity = 10/10 + 2 = 83% + - + - - + - + - + - - - - + - + - + + - - + - Condition Present Condition Absent

17. SnNOUT “When using a test with high sensitivity (Sn), a negative (N) result is useful to rule out (OUT) the condition: SnNOUT”

18. 2 X 2 Contingency Table Reference Standard Condition Absent Condition Present True False + True Positive + False Positive Positive Test Results − False Negative − True Negative Negative Specificity= True Negatives / False Positives + True Negatives

19. Reference Standard Condition Absent Condition Present + True Positive + False Positive Positive Specificity Test Results True False − True Negative - False Negative Negative - - + - - - - - + - - - Specificity = True Negatives / True Negatives + False Positivies

20. Reference Standard Number of subjects: 24 Condition Absent Condition Present 2 False Positive + True Positive - False Positive Positive Specificity Test Results True False - False Negative 10 True Negative + True Negative Negative Specificity = 10/10 + 2 = 83% - - + - - - - - + - - - Condition Present Condition Absent

21. Reference Standard Condition Absent Condition Present 0 True Positive 2 False Positive Positive Specificity Test Results True False 10 True Negative 12 False Negative Negative - - - - - + - - - - - - - - - + - - - - - - - - - Condition Present Condition Absent

22. SpPIN “When using a test with high specificity (Sp), a positive (P) result is useful to rule in (IN) the condition: SpPIN”

23. Sensitivity & Specificity “Given the presence or absence of the condition, what is the probability that this test will give me a correct result?” “How likely it is that a test result will predict the presence or absence of a condition?”

24. Positive & Negative Predictive Values Positive Predictive Value: proportion of subjects with a positive test result who actually have the condition Negative Predictive Value: proportion of subjects with a negative test result who actually do not have the condition Condition Condition Present Not Present + - Test Positive Test Negative + _ True positive False positive True negative False negative “How likely it is that a test result will predict the presence or absence of a condition?”

25. PPV values are highly dependent on prevalence of the condition in the population studied If the sample population studied has a low prevalence of the condition, then the positive predictive value will be lower and the negative predictive value will be higher The reverse holds true for population samples with high prevalence of the condition Positive Predictive Value: proportion of subjects with a positive test result who actually have the condition Negative Predictive Value: proportion of subjects with a negative test result who actually do not have the condition

26. Positive & Negative Predictive Values Calis M, Akgun K, Birtane M, et al. Diagnostic values of clinical diagnostic tests in subacromial impingement syndrome. Ann Rheum Dis. 2000;59:44-47. Reference Standard: Subacromial Injection Test SIS Present SIS Not Present + Pain Positive Pain Negative Positive Predictive Value (PPV): (55/100)= 55% + _ 5 55 45 Test: Hawkins Test (5/50) = 1% True positive False positive Negative Predictive Value (NPV): (15/20)=75% 5 1 15 True negative False negative (15/16) = 94%

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