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