Diffusing capacity in normal-for-age spirometry and spirometric impairments, using reference equations from the global lung function initiative.

Abstract

Reference equations from the Global Lung Function Initiative (GLI) are now available for both spirometry and diffusion. However, respiratory phenotypes defined by GLI-based measures of diffusion have not yet been evaluated in GLI-based normal-for-age spirometry or spirometric impairments. We evaluated cross-sectional data from 2100 Caucasians, aged 40-85 years. GLI-based spirometric categories included normal-for-age and the impairments of restrictive-pattern and three-level severity of airflow-obstruction (mild, moderate, severe). GLI-based diffusion included diffusing capacity of the lung for carbon monoxide (DLCO) and measured components of alveolar volume (VA) and transfer coefficient (KCO): DLCO=[VA]x[KCO]. Using multivariable regression models, adjusted odds ratios (adjORs) for DLCO, VA, and KCO<lower limit of normal (LLN) were calculated for spirometric impairments, relative to normal-for-age spirometry. Relative to normal-for-age spirometry, the restrictive-pattern increased the adjORs (95% confidence intervals) for DLCO and VA<LLN-4.61 (3.62, 5.85) and 15.53 (11.8, 20.4), respectively, but not for KCO<LLN-1.02 (0.79, 1.33). Also relative to normal-for-age spirometry, airflow-obstruction from mild to severe increased the adjORs for DLCO<LLN-from 1.22 (0.80, 1.86) to 6.63 (4.91, 8.95), for VA<LLN-from 1.37 (0.85, 2.18) to 7.01 (5.20, 9.43), and for KCO<LLN-from 2.04 (1.33, 3.14) to 3.03 (2.29, 3.99). Notably, in normal-for-age spirometry, 34.5%, 19.7%, and 25.3% of participants had DLCO, VA, or KCO<LLN, respectively. Abnormal diffusion is most prevalent in spirometric impairments but also occurs in normal-for-age spirometry. These results further inform the respiratory phenotypes of GLI-based spirometric categories and, in turn, the spirometric evaluation of respiratory disease.