You admit a 77 year old female with community acquired pneumonia to the ICU. She was admitted from the emergency department with hypoxemia that is refractory to supplemental oxygen. She is intubated but this required fiber optic intubation because she could not be intubated by direct laryngoscopy. She was placed on antibiotics and required . She does not have other significant past medical history. Two days later, her fraction of inspired oxygen is 40% and she is off positive end expiratory pressure. You decide she is ready to be extubated but the critical care fellow expresses concern that she will fail extubation and will not be able to be reintubated. You state that you would like to use some diagnostic test to help predict if she will be successfully extubated. Your fellow states that the respiratory rate to tidal volume ratio can be used to predict successful extubation and her ratio is 50.
You from the question: In mechanically ventilated patients, can the respiratory rate to tidal volume ratio be used to predict successful extubation? You perform a MEDLINE search using the Mesh terms ‘extubation’ and ‘sensitivity’ and ‘specificity’ and find an article about predictors of successful extubation (NEJM 1991:324:14451450.)
Read the article and decide:
 Are the results of this diagnostic test valid?
 Are the results of this diagnostic test important?
 Can you apply this valid, important evidence about a diagnostic test in caring for your patient?
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; 324:144550.
Are the results of this diagnostic study valid?
 Was there an independent, blind comparison with a reference (“gold”) standard of diagnosis?
 Yes.
 Was the diagnostic test evaluated in an appropriate spectrum of patients (like those in whom it would be used in practice)?
 Yes.
 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 
2579 
begin{align}
mathit{Sensitivity} &=a/(a+c)\
&=731/809\
&=90%
end{align}
begin{align}
mathit{Specificity} &=d/(b+d)\
&=1500/1770\
&=85%
end{align}
begin{align}
text{Likelihood Ratio (positive result) ($LR+$)}&=mathit{sens}/(1mathit{spec})\
&=90%/15%\
&=6
end{align}
begin{align}
text{Likelihood Ratio (negative results) ($LR$)} &=(1mathit{sens})/mathit{spec}\ &=10%/85%\
&=0.12
end{align}
begin{align}
text{Positive Predictive Value}&=a/(a+b)\
&=731/1001\
&=73%
end{align}
begin{align}
text{Negative Predictive Value}&=d/(c+d)\
&=1500/1578\
&=95%
end{align}
begin{align}
text{Pretest Probability (prevalence)}&=(a+c)/(a+b+c+d)\
&=809/2579\
&=32%
end{align}
begin{align}
text{Pretestodds}&=prevalence/(1prevalence)\
&=31%/69%\
&=0.45
end{align}
begin{align}
text{Posttest odds}&=text{Pretest odds}times text{Likelihood Ratio}\
end{align}
begin{align}
text{Posttest Probability}&=text{Posttest odds} /text{(Posttest odds}+ 1)\
end{align}
Your calculations:
Successful extubation  Likelihood Ratio  

Present  Absent  
Diagnostic Test Result  < 80  29  3  
> 80  7  25  
Totals  36  28  64 
begin{align}
mathit{Sensitivity} &=a/(a+c)\
&=29/36 \
&=80%
end{align}
begin{align}
mathit{Specificity} &=d/(b+d) \
&=25/28 \
&=89%
end{align}
begin{align}
text{Likelihood Ratio (positive results) ($LR+$)} &=mathit{sens}/(1mathit{spec}) \
&=7.5
end{align}
begin{align}
text{Likelihood Ratio (negative results) ($LR$)}&=(1mathit{sens})/mathit{spec} \
&=0.22
end{align}
begin{align}
text{Positive Predictive Value}&=a/(a+b) \
&=90%
end{align}
begin{align}
text{Negative Predictive Value}&=d/(c+d) \
&=79%
end{align}
begin{align}
text{Pretest Probability ($prevalence$)} &=(a+c)/(a+b+c+d) \
&=56%
end{align}
begin{align}
mathit{Pretestodds}&=mathit{prevalence}/(1mathit{prevalence})\
&=1.28
end{align}
begin{align}
text{Posttest odds} &= text{Pretest odds}times text{Likelihood Ratio}\
&=9.6
end{align}
begin{align}
text{Posttest Probability}&=text{Posttest odds}/(text{Posttest odds} + 1) \
&=9.6/10.6\
&=91%
end{align}
Can you apply this valid, important evidence about a diagnostic test in caring for your patient?
 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?
 Can you generate a clinically sensible estimate of your patient’s pretest probability (from practice data, from personal experience, from the report itself, or from clinical speculation)
 Approximately 70%
 Will the resulting posttest probabilities affect your management and help your patient? (Could it move you across a testtreatment threshold?; Would your patient be a willing partner in carrying it out?)
 Her respiratory rate to tidal volume ratio of 50 gives her a posttest probability of 95% and this crosses my treatment threshold of 90%
 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; 25:151421.
This article provides an excellent review of how to evaluate an article about a diagnostic test.
Successful Extubation: How to diagnose
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; 324:144550.
Clinical Question
In mechanically ventilated patients, can the respiratory rate to tidal volume ratio be used to predict successful extubation?
Search Terms
‘extubation’ and ‘sensitivity’ and ‘specificity’
The Study
 Target disorder and gold standard; Successful extubation
 Study setting: mechanically ventilated patients in an an ICU
The Evidence
Respiratory rate to tidal volume ratio  Successful extubation  Failed extubation  Likelihood ratio 

< 80  29/32  3/32  7.53 
80100  6/12  6/12  0.77 
> 100  1/20  19/20  0.04 
Totals  36  28 
Comments
 Study designed to evaluate weaning but outcome was successful extubation.
 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; 25:151421.
This article provides an excellent review of how to evaluate an article about a diagnostic test.