Diagnosis: EBM Approach
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Transcript of Diagnosis: EBM Approach
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Diagnosis: EBM Approach
Michael Brown MD
Grand Rapids MERC/ Michigan State
University
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Scenario
1 day colicky pain with nausea diffuse to RLQ mild tenderness, T 37
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Step 1: Clinical Question
In the patient presenting to the ED with suspected appendicitis, what is the accuracy of helical CT ?
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Step 2: Search
MeSH Browser– appendicitis AND– computerized tomography AND– sensitivity and specificity
Clinical Query– diagnosis
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Step 3: Critical Appraisal
Internal Validity Results (focus today)
– even if critical appraisal not your bag
External Validity
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Evidenced-based Medicine
stresses methodology de-emphasizes statistics simplify: NNT, LR
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Likelihood Ratio: How to use
How to calculate
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Examples
Appendicitis Pulmonary embolism
– JAMA series– current slant
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Threshold Approach to Clinical Decision Making
Treatment threshold– if above, start therapy
Test threshold– if below, no further testing
Pauker NEJM 1980
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Diagnostic Testing
Treatment threshold for PE?– If above: heparin
Test threshold for PE?– If below: discharge home
If between?– Further testing
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Test/Treatment Threshold
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prior probability
post-test probLR
(prevalence) (predictive value)
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Test/Treatment Threshold
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Pretest Probability
experience in your setting– patient population
prevalence of condition in literature– Oxford web site
scoring systems
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Estimate Pretest Probability for PE
history risk factors physical exam initial screening tests
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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
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Bayesian Analysis?
Thomas Bayes 1702-1761 English clergyman Doctrine of Chances
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Bayesian Analysis
pretest probability– prevalence
LR for diagnostic test result post-test probability
– predictive value
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Interpretation
convert pretest prob to odds odds x LR = post-test odds convert odds back to prob
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Interpretation
convert pretest prob to odds odds x LR = post-test odds convert odds back to prob
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Fagan Nomogram
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Effect on pretest probability:
>10 or <0.1 large changes 5-10 and 0.1-0.2 moderate approach 1 no effect
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Advantages of LR:
combines sensitivity and specificity interpret test result on individual patient multiple cut-offs sequential testing
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Shortcut: LR for + test =
sensitivity
1 - specificity
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Calculate: LR=
prob (test result) with disease
prob (test result) without disease
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2 x 2 Table
Target Disorder Totals
Present Absent
DiagnosticTest Result
Positive a b a+b
Negative c d c+d
Totals a+c b+d a+b+c+d
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CT and Appendicitis
Target Disorder Totals
Present Absent
DiagnosticTest Result
Positive 29 4 33
Negative 1 66 67
Totals 30 70 100
Funaki et al
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CT and Appendicitis
probability of + CT with appendicitis
29/30 = .97
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CT and Appendicitis
Target Disorder Totals
Present Absent
DiagnosticTest Result
Positive 29 4 33
Negative 1 66 67
Totals 30 70 100
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CT and Appendicitis
Likelihood of + CT with appendicitis
29/30 = .97 Likelihood of +CT without appendicitis
4/70 = .057 LR for + CT = 17
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Effect on pretest probability:
>10 or <0.1 large changes 5-10 and 0.1-0.2 moderate approach 1 no effect
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Scenario
1 day colicky pain with nausea diffuse to RLQ mild tenderness, T 37
Pretest probability 30% – range 20 - 40%
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Helical CT
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Helical CT
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Helical CT
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Effect on pretest probability:
>10 or <0.1 large changes 5-10 and 0.1-0.2 moderate approach 1 no effect
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Advantages of LR:
combines sensitivity and specificity interpret test result on individual patient multiple cut-offs
– don’t have to lump!
sequential testing
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Multiple cut-offs
appendicitis No disease Total
CT positive
CT equivocal a b
CT negative
Total x y
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Calculate: LR=
prob equivocal CT with disease
prob equivocal CT without disease
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Multiple cut-offs
appendicitis No disease Total
CT positive
CT equivocal a b
CT negative
Total x y
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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
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Sequential Testing
post-test probability 1st test new pretest probability for 2nd test assume independence
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Helical CT : Diagnosis of PE
CT +
CT -
LR 8
LR .2
Rathbun, 2000
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ELISA D-dimer: Diagnosis of PE
> 500
<500
LR 2
LR .1
Brown, Bermingham 2001
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Helical CT
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D-dimer
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Test/Treatment Threshold
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Questions?
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Scenario
1 day colicky pain with nausea diffuse to RLQ mild tenderness, T 37
Pretest probability 30%
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Discussion: CT and appy
Internal Validity Results External Validity
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CT and Appendicitis
Flaws?– Minor– Major– Fatal
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CT and Appendicitis (Rao)
Target Disorder
Totals
Present Absent
Diagnostic Test Result
Positive
52 1 53
Negative
1 46 47
Totals 53 47 100
Rao et al
+LR = .98/.02 = 46-LR = .019/.98= .02
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CT and Appendicitis (Funaki)
Target Disorder Totals
Present Absent
DiagnosticTest Result
Positive 29 4 33
Negative 1 66 67
Totals 30 70 100
Funaki et al
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CT and Appendicitis
Likelihood of + CT with appendicitis
29/30 = .97 Likelihood of +CT without appendicitis
4/70 = .057 LR for + CT = 17
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Funaki: CT and Appendicitis
LR for a positive CT 17 LR for a negative CT 0.03
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CT and Appendicitis
Likelihood of + CT with appendicitis
29/30 = .97 Likelihood of +CT without appendicitis
4/70 = .057 LR for + CT = 17
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Helical CT
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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
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Combinations (LRxLR)
D-dimer <500, CT - LR .02