Diagnosis: EBM Approach

1. Diagnosis: EBM Approach Michael Brown MD Grand Rapids MERC/ Michigan State University

2. Scenario • 1 day colicky pain with nausea • diffuse to RLQ • mild tenderness, T 37

3. Step 1: Clinical Question • In the patient presenting to the ED with suspected appendicitis, what is the accuracy of helical CT ?

4. Step 2: Search • MeSH Browser • appendicitis AND • computerized tomography AND • sensitivity and specificity • Clinical Query • diagnosis

7. Step 3: Critical Appraisal • Internal Validity • Results (focus today) • even if critical appraisal not your bag • External Validity

8. Evidenced-based Medicine • stresses methodology • de-emphasizes statistics • simplify: NNT, LR

9. Likelihood Ratio: How to use How to calculate

10. Examples • Appendicitis • Pulmonary embolism • JAMA series • current slant

11. Threshold Approach to Clinical Decision Making • Treatment threshold • if above, start therapy • Test threshold • if below, no further testing Pauker NEJM 1980

12. Diagnostic Testing • Treatment threshold for PE? • If above: heparin • Test threshold for PE? • If below: discharge home • If between? • Further testing

13. Test/Treatment Threshold

14. LR post-test prob prior probability (predictive value) (prevalence)

15. Test/Treatment Threshold

16. Pretest Probability • experience in your setting • patient population • prevalence of condition in literature • Oxford web site • scoring systems

17. Estimate Pretest Probability for PE • history • risk factors • physical exam • initial screening tests

18. Estimate Pretest Probability for PE • Not exact science • usually a range 40-60% • low, intermediate, high • done daily in clinical practice • clinical prediction rules • physicians estimate very close Wicki 2001

19. Bayesian Analysis? • Thomas Bayes 1702-1761 • English clergyman • Doctrine of Chances

20. Bayesian Analysis • pretest probability • prevalence • LR for diagnostic test result • post-test probability • predictive value

21. Interpretation • convert pretest prob to odds • odds x LR = post-test odds • convert odds back to prob

22. Interpretation • convert pretest prob to odds • odds x LR = post-test odds • convert odds back to prob

23. Fagan Nomogram

24. Effect on pretest probability: • >10 or <0.1 large changes • 5-10 and 0.1-0.2 moderate • approach 1 no effect

25. Advantages of LR: • combines sensitivity and specificity • interpret test result on individual patient • multiple cut-offs • sequential testing

26. Shortcut: LR for + test = sensitivity 1 - specificity

27. Calculate: LR= prob (test result) with disease prob (test result) without disease

28. 2 x 2 Table

29. CT and Appendicitis Funaki et al

30. CT and Appendicitis • probability of + CT with appendicitis 29/30 = .97

31. CT and Appendicitis

32. CT and Appendicitis • Likelihood of + CT with appendicitis 29/30 = .97 • Likelihood of +CT without appendicitis 4/70 = .057 • LR for + CT = 17

33. Effect on pretest probability: • >10 or <0.1 large changes • 5-10 and 0.1-0.2 moderate • approach 1 no effect

34. Scenario • 1 day colicky pain with nausea • diffuse to RLQ • mild tenderness, T 37 • Pretest probability 30% • range 20 - 40%

35. Helical CT

36. Helical CT

37. Helical CT

38. Effect on pretest probability: • >10 or <0.1 large changes • 5-10 and 0.1-0.2 moderate • approach 1 no effect

39. Advantages of LR: • combines sensitivity and specificity • interpret test result on individual patient • multiple cut-offs • don’t have to lump! • sequential testing

40. Multiple cut-offs

41. Calculate: LR= prob equivocal CT with disease prob equivocal CT without disease

42. Multiple cut-offs

43. normal low prob intermediate prob high prob LR 0.1 LR 0.4 LR 1 LR 18 Multiple cut-offs: V/Q scan JAMA series

44. Sequential Testing • post-test probability 1st test • new pretest probability for 2nd test • assume independence

45. CT + CT - LR 8 LR .2 Helical CT : Diagnosis of PE Rathbun, 2000

46. > 500 <500 LR 2 LR .1 ELISA D-dimer: Diagnosis of PE Brown, Bermingham 2001

48. Helical CT

49. D-dimer

50. Test/Treatment Threshold