Evidence based medicine

1. Evidence based medicine Diagnostic tests Ross Lawrenson

2. Diagnostic tests • When looking at a paper about a diagnostic test we ask ourselves three questions.

3. Diagnostic tests • Is this test useful?

4. Diagnostic tests • Is this test useful? • Is it reliable?

5. Diagnostic tests • Is this test useful? • Is it reliable? • Is it valid?

6. Is this test useful? • The test should have been researched in a study populationrelevantto the individual or population in whom it is to be used.

7. Reliability • Reliability refers to the repeatability or reproducibility of a test. • It can be assessed by repeating the test using the same or different observers.

8. Validity • Relates to whether the test measures what it purports to measure. Is the result true?

9. Validity • For example if you measure blood pressure in an obese patient and use a cuff that is too small you are likely to get a falsely high reading. The reading maybe reliable (you get the same blood pressure if you do it again) but it lacks validity.

10. Sensitivity and specificity

11. Sensitivity and specificity Disease Healthy Total Test + a b a+b Test - c d c+d Total a+c b+d

12. Sensitivity and specificity

13. Sensitivity • The probability that the test will be positive if the disease is present • = a/a+c

14. Sensitivity • The probability that the test will be positive if the disease is present • = a/a+c • A sensitive test is likely to also record a number of false positive tests

15. Sensitivity If the cut off point of this test is set low then it will be sensitive (all patients with disease will test positive) but there will also be a number of false positives Healthy Diseased

16. Specificity • Theprobability that the test will be negative if the disease is truly absent. • d/b+d

17. Specificity • Theprobability that the test will be negative if the disease is truly absent. • d/b+d • In this situation there is a high likelihood of false negatives.

23. Accuracy of the test (a+d)/(a+b+c+d)

24. Example 5000 women underwent a test for blood glucose at 24 weeks following a glucose load. 243 women were found to have a blood glucose greater than 6.8 mmol/L and were referred for an OGTT. 186 were found to have gestational diabetes. Four women who initially had tested negative were diagnosed as having diabetes later in their pregnancy.

25. Example Prevalence Sensitivity Specificity Positive predictive value Negative predictive value Likelihood ratio + test Likelihood ratio - test Accuracy

26. Diabetes No diabetes Total Positive 186 57 243 Negative 4 4753 4757 Total 190 4810 5000

27. Example Prevalence 190/5000 Sensitivity 186/190 Specificity 4753/4810 Positive predictive value 186/243 Negative predictive value 4753/4757 Likelihood ratio + test (186/190)/(57/4810) Likelihood ratio - test (4/190)/(4753/4810) Accuracy 186+4753/5000

30. Gold standard . .

31. Gold standard • The gold standard is the test or battery of tests that will most accurately diagnose a particular disease or condition. • Thus traditionally the OGTT has been seen as the gold standard when testing for diabetes. Other diagnostic tests may have a gold standard that is too expensive or invasive for routine use e.g. fluoroscein angiography for diabetic retinopathy. • Sometimes the gold standard is a battery of tests or symptoms e.g. the Jones criteria for rheumatic fever

37. Percent agreement Abnormal Suspect Normal Abnormal A B C Suspect D E F Normal G H I Percent agreement = (A+E+I) / Total X100

39. Percent agreement Melanoma Indeterminate Benign Melanoma 10 1 10 Indeterminate 0 0 0 Benign 0 0 16 Percent agreement = (10+0+16)/37 X100 = 70 %

40. KAPPA Second Exam Normal Retinopathy Total First Normal 46 10 56 Exam Retinopathy 12 32 44 Total 58 42 100 Observed agreement = 46 + 32/100 = 78%

41. KAPPA Second Exam Normal Retinopathy Total First Normal 58%x56 42%x56 56 Exam Retinopathy 58%x44 42%x44 44 Total 58 42 100

42. KAPPA Second Exam Normal Retinopathy Total First Normal 32.5 23.5 Exam Retinopathy 25.5 18.5 Total Agreement expected by chance =32.5+18.5/100 =51%