title: Comparison of the lower confidence limit to the fixed-percentage method for assessing airway obstruction in routine clinical practice.
abstract: BACKGROUND
  Although the statistically derived lower limit of normal (LLN) for the ratio of FEV(1) to FVC is considered superior to a fixed cutoff value (such as 0.70) for diagnosing airway obstruction, the fixed-cutoff method continues to be used and advocated.
  
  
  OBJECTIVE
  To evaluate the misclassification of spirometrically determined airway obstruction arising from the use of the fixed-percent method, in comparison to the LLN method for FEV(1)/FVC.
  
  
  METHODS
  We reviewed 27,307 spirometry records from adult men, and diagnosed airway obstruction based  on the LLN (predicted value minus 1.645 times the standard error of estimate from a north Indian reference equation for FEV(1)/FVC) and based on a fixed cutoff of 0.70. We computed agreement and discordance between the two methods, and determined the sensitivity, specificity, and  predictive values of the fixed-percent method in identifying true obstruction.
  
  
  RESULTS
  The results were discordant in 1,622 subjects (6%). Overall agreement between the two methods was good (kappa estimate 0.869), but worsened considerably with advancing age. 1,290 subjects (5%) who were  deemed normal with the LLN method were diagnosed as having airway obstruction with the fixed-percentage method. Overall the sensitivity, specificity, and positive predictive value of the fixed-percentage method were 0.963, 0.929, and 0.871, respectively. Specificity and positive predictive  value decreased sharply with advancing age.
  
  
  CONCLUSIONS
  The negative age-dependence of FEV(1)/FVC results in over-diagnosis of airway obstruction in middle-aged and elderly men, and under-diagnosis in young men, with the fixed-percentage method. Airway obstruction should be assessed  with the LLN of FEV(1)/FVC, with the LLN derived from appropriate reference equations.
authors: Department of Pulmonary Medicine, Postgraduate Institute of Medical Education and Research, Chandigarh, India. ashutosh@indiachest.org
authors: Ashutosh N
authors: AN
authors: Aggarwal
authors: Dheeraj
authors: D
authors: Gupta
authors: Ritesh
authors: R
authors: Agarwal
authors: Surinder K
authors: SK
authors: Jindal