Comparison of the lower confidence limit to the fixed-percentage method for assessing airway obstruction in routine clinical practice.

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.