As Dr. O'brien eloquently described in this post, many people misunderstand the Yang-Tobin (f/Vt) index as being a "weaning parameter" that is predictive of extubation success. Far from that, it's sensitivity and specificity and resultant ROC curve relate to the ability of f/Vt after one minute of spontaneous ventilation to predict the success of a prolonged (~ one hour) spontaneous breathing trial. But why would I want to predict the result of a test (the SBT), and introduce error, when I can just do the test and get the result an hour later? It makes absolutely no sense. What we want is a parameter that predicts extubation success. But we don't have that, and we probably will never have that.In order to determine the sensitivity and specificity of a test for extubation success, we will need to ascertain the outcome in all patients regardless of their performance on the test of interest. That means we would have to extubate patients that failed the weaning parameter test. In the original Yang & Tobin article, their cohort consisted of 100 patients. 60(%) of the 100 were said to have passed the weaning test and were extubated, and 40(%) failed and were not extubated. (There is some over-simplification here based on how Yang & Tobin classified and reported events - its not at all transparent in their article - the data to resolve the issues are not reported and the differences are likely to be small. Suffice it to say that about 60% of their patients were successfully weaned and the remainder were not.) Let's try to construct a 2x2 table to determine the sensitivity and specificity of a weaning parameter using a population like theirs. The top row of the 2x2 table would look something like this, assuming an 85% extubation success rate - that is, of the 60 patients with a positive or "passing" SBT score (based on whatever parameter), all were extubated and the positive predictive value of the test is 85% (the actual rate of reintubation in patients with a passing weaning test is not reported, so this is a guess):
