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FEV₁/FVC Ratio of 70% Misclassifies Patients With Obstruction at the Extremes of Age^*

^* From the Department of Medicine (Drs. Roberts, Farber, and Knox), Division of Pulmonary, Critical Care, and Occupational Medicine, Indiana University School of Medicine, Indianapolis, IN; and the Center for Biostatistics (Mr. Phillips) and Department of Medicine (Drs. Bhatt, Mastronarde, and Wood), Division of Pulmonary, Critical Care, and Sleep Medicine, The Ohio State University Medical Center, The Ohio State University, Columbus, OH. The authors have reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

Correspondence to: Karen L. Wood, MD, 201 Davis Heart and Lung Institute, 473 W Twelfth Ave, Columbus, OH 43210; e-mail: wood.555{at}osu.edu

Abstract

Background: The American Thoracic Society recommends using the lower limit of normal (LLN) method to diagnose obstructive lung disease. However, few studies have investigated the clinical relevance of these recommendations. We compared the LLN derived from available data sets to a fixed ratio (FEV₁/FVC, < 75% or 70%) and also to the FEV₁/FVC percent predicted ratio to determine the impact of changing the FEV₁/FVC "cutoff" on the spirometric diagnosis of obstructive lung disease.

Methods: FEV₁, FVC, FEV₁/FVC ratio, age, race, sex, height, and weight were recorded from 1,503 pulmonary function tests. Predicted values were calculated using the Third National Health and Nutrition Examination Study data set (Hankinson), and reference values from studies by Crapo, Knudson, and Morris. In addition, the LLN of the FEV₁/FVC ratio was calculated for the Hankinson and Crapo reference values.

Results: The number of studies interpreted as obstructed varied from 37% using the Hankinson data set to 55% using the 75% fixed ratio method. Comparing the LLN method vs the 70% fixed ratio method resulted in 7.5% (Hankinson LLN vs 70% fixed) and 6.9% (Crapo LLN vs 70% fixed), which were discordant results. Age was the strongest predictor of discordance, and 16% of subjects > 74 years of age had discordant results comparing Hankinson values to the 70% fixed method.

Conclusion: At the extremes of age and height, a large number of spirometry test results will be interpreted as showing an obstructive defect if a 70% fixed ratio method is used for interpretation compared with the LLN derived from the Hankinson data set.

Key Words: airway obstruction • lower limit of normal • pulmonary function testing • spirometry

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