Authors: Aggarwal, Ashutosh N; Gupta, Dheeraj; Agarwal, Ritesh; Jindal, Surinder K

Source: Respiratory Care, Volume 56, Number 11, November 2011 , pp. 1778-1784(7)

Publisher: The Journal Respiratory Care Company

Abstract:

BACKGROUND: Although the statistically derived lower limit of normal (LLN) for the ratio of FEV₁ 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 FEV₁/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 FEV₁/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 FEV₁/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 FEV₁/FVC, with the LLN derived from appropriate reference equations.

Keywords: airway obstruction; India; obstructive lung diseases; predictive value of tests; reference standards; spirometry

Document Type: Research article

DOI: http://dx.doi.org/10.4187/respcare.01160

Affiliations: 1: Department of Pulmonary Medicine, Postgraduate Institute of Medical Education and Research, Chandigarh, India

Publication date: 2011-11-01

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