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Interpreting lung function data using 80% predicted and fixed thresholds identifies patients at increased risk of mortality.

BACKGROUND
The GOLD (Global Initiative for Chronic Obstructive Lung Disease) stages for COPD use a fixed ratio of the postbronchodilator FEV(1)/FVC ratio of 0.70 as a threshold to define obstruction. Others advocate using the lower limit of normal (LLN) for the FEV(1)/FVC ratio, FEV(1), and FVC to define abnormality. This study investigated mortality in a representative sample of the US adult population with COPD by comparing abnormality determined using GOLD criteria to that determined using LLN criteria.


METHODS
We used baseline data from the Third National Health and Nutrition Examination Survey and follow-up mortality data. We classified subjects as obstructed, restricted, or normal based on GOLD vs LLN criteria and used Cox proportional hazards models to determine the relation between lung function impairment and mortality, adjusting for covariates.


RESULTS
The study sample included 13,847 subjects, of whom 3,774 died during the follow-up period. Of subjects classified as obstructed and restricted using GOLD criteria, 20.9% and 18.0%, respectively, were classified as normal using LLN criteria. Compared with people with normal lung function, mortality was increased in the obstructed (hazard ratio, 1.46; 95% CI, 1.21-1.86) and restricted (hazard ratio, 1.94; 95% CI, 1.58-2.39) subjects classified as normal using the LLN.


CONCLUSIONS
In the nationally representative Third National Health and Nutrition Examination Survey data, subjects classified as normal using LLN criteria but obstructed or restricted using GOLD criteria have a higher risk of mortality.