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Diagnosis: EBM Approach

Diagnosis: EBM Approach

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Diagnosis: EBM Approach

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  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

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

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

  7. Likelihood Ratio: How to use How to calculate

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

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

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

  11. Test/Treatment Threshold

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

  13. Test/Treatment Threshold

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

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

  16. 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

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

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

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

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

  21. Fagan Nomogram

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

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

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

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

  26. 2 x 2 Table

  27. CT and Appendicitis Funaki et al

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

  29. CT and Appendicitis

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

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

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

  33. Helical CT

  34. Helical CT

  35. Helical CT

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

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

  38. Multiple cut-offs

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

  40. Multiple cut-offs

  41. 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

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

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

  44. > 500 <500 LR 2 LR .1 ELISA D-dimer: Diagnosis of PE Brown, Bermingham 2001

  45. Helical CT

  46. D-dimer

  47. Test/Treatment Threshold