Diagnostic Tests Results of a Screening Test 5
Description: Diagnostic Tests Results of a Screening Test 5 Sensitivity Sensitivity True positives Affected persons The sensitivity of a test in the ability of the test to identify correctly affected individuals Proportion of persons testing
Related Topics
Download Presentation
"Diagnostic Tests Results of a Screening Test 5" is the property of its rightful owner. Permission is granted to download and print the materials on this website for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Presentation Transcript
slide1. Diagnostic Tests<br>
slide5. Results of a Screening Test 5<br>
slide6. Sensitivity Sensitivity = True positives / Affected persons The sensitivity of a test in the ability of the test to identify correctly affected individuals
Proportion of persons testing positive among affected individuals<br>
slide7. Measures of Test Validity 7<br>
slide8. Example Sensitivity = 148 / (150) = 98%<br>
slide9. Specificity Specificity = True negatives / Non-affected persons The specificity of a test is the ability of the test to identify correctly non-affected individuals
Proportion of person testing negative among non affected individuals<br>
slide10. Example Specificity = 188 / 200 = 94%<br>
slide11. Identifying the cut-off to use with a test on the basis of panel analysis: Ideal case Cut-off 0 5 10 15 20 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Possible values of the test Number of tests Sick Well<br>
slide12. Identifying the cut-off to use with a test on the basis of panel analysis: Real case Cut-off 0 5 10 15 20 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Possible values of the test Number of tests Sick Well<br>
slide13. Predictive value of a positive test = True positives / Persons testing positive The predictive value of a positive test is the probability that an individual testing positive is truly affected
Proportion of affected persons among those testing positive<br>
slide14. How is the test doing in a real population? The test is now used in a real population
This population is made of
Affected individuals
Non-affected individuals
The proportion of affected individuals is the prevalence<br>
slide15. Results of a Screening Test 15<br>
slide16. PVP = A / (A+B)<br>
slide17. Predictive value of a negative test = True negatives / Persons testing negative The predictive value of a negative test is the probability that an individual testing negative is truly non-affected
Proportion of non-affected persons among those testing negative<br>
slide18. Evaluation of Screening Program 18<br>
slide19. Accuracy = (TP+TN)/ Total result
Tells how often you’re right
Best test: has high level of sensitivity and specificity<br>
slide20. Suitable Test (cont’d) Validity (Accuracy):
The ability of a test to give a true measure.
Can be evaluated only if an accepted and independent method for confirming the test measurement exists.
Valid Test: Correctly classifies people with disease as positive and people without disease as negative. 20<br>
slide21. Suitable Test (cont’d) Reliability (Precision):
The ability of a measuring instrument to give consistent results on repeated trials.
Repeated measurement reliability--the degree of consistency among repeated measurements of the same individual on more than one occasion. 21<br>
slide22. Interrelationships Between Reliability and Validity It is possible for a measure to be highly reliable but invalid.
It is not possible for a measure to be valid but unreliable. 22<br>
slide23. Representation of Reliability and Validity 23<br>
slide25. ADPKD screening of pts with positive family hx of ADPKD (Not Real Data) Patients assigned to screening or usual care. Screening consisted of yearly renal US and physical exam. Five years of follow‑up produced these results: ADPKD Screening Test Result 25<br>
slide26. Suitable Test Sensitivity = 132/177 = 74.6%
Specificity = 63,650/64,633 = 98.5%
Interpretation: The screening was very good at picking out the patients who did not have ADPKD (see specificity) but it missed 25% of the patients who did have ADPKD (see sensitivity).
To measure sensitivity and specificity you can wait for disease to develop (as above) or you can measure the results of the screening test against the outcome of another screening or diagnostic test (the Gold Standard). 26<br>
slide27. Evaluation of Screening Program: Feasibility Measures Acceptability, cost, predictive value of a positive test (PV+), predictive value of a negative test (PV-) Screening Test
Result Disease Status 27<br>
slide28. Evaluation of Screening Program 28<br>
slide29. ADPKD screening of pts with positive family hx of ADPKD (Not Real Data) 29<br>
slide30. PV+ = 132/1115 = 11.8% PV- = 63,650/63,695 = 99.9%<br>
slide31. Evaluation of Screening Program PV will increase when sensitivity, specificity, and disease prevalence increases.
For example, PV+ will increase if you perform ADPKD screening on higher risk population (i.e. people with a family history of ADPKD) 31<br>
slide33. END!?<br>
slide5. Results of a Screening Test 5<br>
slide6. Sensitivity Sensitivity = True positives / Affected persons The sensitivity of a test in the ability of the test to identify correctly affected individuals
Proportion of persons testing positive among affected individuals<br>
slide7. Measures of Test Validity 7<br>
slide8. Example Sensitivity = 148 / (150) = 98%<br>
slide9. Specificity Specificity = True negatives / Non-affected persons The specificity of a test is the ability of the test to identify correctly non-affected individuals
Proportion of person testing negative among non affected individuals<br>
slide10. Example Specificity = 188 / 200 = 94%<br>
slide11. Identifying the cut-off to use with a test on the basis of panel analysis: Ideal case Cut-off 0 5 10 15 20 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Possible values of the test Number of tests Sick Well<br>
slide12. Identifying the cut-off to use with a test on the basis of panel analysis: Real case Cut-off 0 5 10 15 20 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Possible values of the test Number of tests Sick Well<br>
slide13. Predictive value of a positive test = True positives / Persons testing positive The predictive value of a positive test is the probability that an individual testing positive is truly affected
Proportion of affected persons among those testing positive<br>
slide14. How is the test doing in a real population? The test is now used in a real population
This population is made of
Affected individuals
Non-affected individuals
The proportion of affected individuals is the prevalence<br>
slide15. Results of a Screening Test 15<br>
slide16. PVP = A / (A+B)<br>
slide17. Predictive value of a negative test = True negatives / Persons testing negative The predictive value of a negative test is the probability that an individual testing negative is truly non-affected
Proportion of non-affected persons among those testing negative<br>
slide18. Evaluation of Screening Program 18<br>
slide19. Accuracy = (TP+TN)/ Total result
Tells how often you’re right
Best test: has high level of sensitivity and specificity<br>
slide20. Suitable Test (cont’d) Validity (Accuracy):
The ability of a test to give a true measure.
Can be evaluated only if an accepted and independent method for confirming the test measurement exists.
Valid Test: Correctly classifies people with disease as positive and people without disease as negative. 20<br>
slide21. Suitable Test (cont’d) Reliability (Precision):
The ability of a measuring instrument to give consistent results on repeated trials.
Repeated measurement reliability--the degree of consistency among repeated measurements of the same individual on more than one occasion. 21<br>
slide22. Interrelationships Between Reliability and Validity It is possible for a measure to be highly reliable but invalid.
It is not possible for a measure to be valid but unreliable. 22<br>
slide23. Representation of Reliability and Validity 23<br>
slide25. ADPKD screening of pts with positive family hx of ADPKD (Not Real Data) Patients assigned to screening or usual care. Screening consisted of yearly renal US and physical exam. Five years of follow‑up produced these results: ADPKD Screening Test Result 25<br>
slide26. Suitable Test Sensitivity = 132/177 = 74.6%
Specificity = 63,650/64,633 = 98.5%
Interpretation: The screening was very good at picking out the patients who did not have ADPKD (see specificity) but it missed 25% of the patients who did have ADPKD (see sensitivity).
To measure sensitivity and specificity you can wait for disease to develop (as above) or you can measure the results of the screening test against the outcome of another screening or diagnostic test (the Gold Standard). 26<br>
slide27. Evaluation of Screening Program: Feasibility Measures Acceptability, cost, predictive value of a positive test (PV+), predictive value of a negative test (PV-) Screening Test
Result Disease Status 27<br>
slide28. Evaluation of Screening Program 28<br>
slide29. ADPKD screening of pts with positive family hx of ADPKD (Not Real Data) 29<br>
slide30. PV+ = 132/1115 = 11.8% PV- = 63,650/63,695 = 99.9%<br>
slide31. Evaluation of Screening Program PV will increase when sensitivity, specificity, and disease prevalence increases.
For example, PV+ will increase if you perform ADPKD screening on higher risk population (i.e. people with a family history of ADPKD) 31<br>
slide33. END!?<br>