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

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 <0.1large 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. 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 <0.1large 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

38. Effect on pretest probability: >10 or <0.1large 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 appendicitisNo diseaseTotal CT positive CT equivocalab CT negative Totalxy

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

42. Multiple cut-offs appendicitisNo diseaseTotal CT positive CT equivocalab CT negative Totalxy

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

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

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

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

48. Helical CT

49. D-dimer

50. Test/Treatment Threshold

51. Questions?

52. Scenario 1 day colicky pain with nausea diffuse to RLQ mild tenderness, T 37 Pretest probability 30%

53. Discussion: CT and appy Internal Validity Results External Validity

54. CT and Appendicitis Flaws? – Minor – Major – Fatal

55. CT and Appendicitis (Rao) Rao et al +LR =.98/.02 = 46 -LR =.019/.98=.02

56. CT and Appendicitis (Funaki) Funaki et al

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

58. Funaki: CT and Appendicitis LR for a positive CT 17 LR for a negative CT 0.03

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

60. Helical CT

61. Sources of LR: PE: + CT LR = 8 - CT LR =.2 – meta-analysis: Rathbun et al Pharyngitis: neg rapid strep - LR = 0.2 – hosptial data Alcoholism: CAGE >3 LR = 250 – web

62. Combinations (LRxLR) D-dimer <500, CT - LR.02