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Centre for Evidence-
Based Medicine

Completed Diagnosis Worksheet for Critical Care Medicine

Citation

Yang KL, Tobin MJ.
A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation.
N Engl J Med 1991; 324:1445-50.

Are the results of this diagnostic study valid?

  1. Was there an independent, blind comparison with a reference ("gold") standard of diagnosis?
    Yes.
  2. Was the diagnostic test evaluated in an appropriate spectrum of patients (like those in whom it would be used in practice)?
    Yes.
  3. Was the reference standard applied regardless of the diagnostic test result?
    Yes.

Are the valid results of this randomised trial important?

Sample calculations:

Target Disorder (iron deficiency anaemia) Totals
Present Absent
Diagnostic Test Result (serum ferritin) Positive (< 65 mmol/L)

731

a

270

b

1001

a + b

Negative (≥ 75 mmol/L)

78

c

1500

d

1578

c + d

Totals

a + c

809

b + d

1770

a + b + c + d

2579

Sensitivity = a/(a+c)
= 731/809
= 90%

Specificity = d/(b+d)
= 1500/1770
= 85%

Likelihood Ratio (positive result) (LR+) = sens/(1-spec)
= 90%/15%
= 6

Likelihood Ratio (negative results) (LR-) = (1-sens)/spec
= 10%/85%
= 0.12

Positive Predictive Value = a/(a+b)
= 731/1001
= 73%

Negative Predictive Value = d/(c+d)
= 1500/1578
= 95%

Pre-test Probability (prevalence) = (a+c)/(a+b+c+d)
= 809/2579
= 32%

Pre-test-odds = prevalence/(1-prevalence)
= 31%/69%
= 0.45

Post-test odds = Pre-test odds x Likelihood Ratio

Post-test Probability = Post-test odds/(Post-test odds + 1)

Your calculations:

Successful extubation Likelihood Ratio
Present Absent
Diagnostic Test Result < 80 29 3
> 80 7 25
Totals 36 28 64

Sensitivity = a/(a+c)
= 29/36
= 80%

Specificity = d/(b+d)
= 25/28
= 89%

Likelihood Ratio (positive result) (LR+) = sens/(1-spec)
= 7.5

Likelihood Ratio (negative results) (LR-) = (1-sens)/spec
= 0.22

Positive Predictive Value = a/(a+b)
= 90%

Negative Predictive Value = d/(c+d)
= 79%

Pre-test Probability (prevalence) = (a+c)/(a+b+c+d)
= 56%

Pre-test-odds = prevalence/(1-prevalence)
= 1.28

Post-test odds = Pre-test odds x Likelihood Ratio
= 9.6

Post-test Probability = Post-test odds/(Post-test odds + 1)
= 9.6/10.6
= 91%

Can you apply this valid, important evidence about a diagnostic test in caring for your patient?

  1. Is the diagnostic test available, affordable, accurate, and precise in your setting?
    Yes, the authors gave a detailed description of how the test was performed
  2. Can you generate a clinically sensible estimate of your patient's pre-test probability (from practice data, from personal experience, from the report itself, or from clinical speculation)
    Approximately 70%
  3. Will the resulting post-test probabilities affect your management and help your patient? (Could it move you across a test-treatment threshold?; Would your patient be a willing partner in carrying it out?)
    Her respiratory rate to tidal volume ratio of 50 gives her a post-test probability of 95% and this crosses my treatment threshold of 90%
  4. Would the consequences of the test help your patient?
    Yes, since she is a difficult intubation, we want to minimise our chance of a false positive (we state she could be successfully extubated but she fails).

Additional Notes

Jaeschke RZ, Meade MO, Guyatt GH, Keenan SP, Cook DJ.
How to use diagnostic test articles in the intensive care unit: Diagnosing Weanability using f/VT.
Crit Care Med 1997; 25:1514-21.

This article provides an excellent review of how to evaluate an article about a diagnostic test.

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