The Prognostic Performance of Lung Diffusing Capacity in Preserved Ratio Impaired Spirometry: An Observational Cohort Study

PURPOSE: Similar to chronic obstructive pulmonary disease (COPD), the diffusing capacity of the lung (D(LCO)) might be decreased and associated with poor prognosis in preserved ratio impaired spirometry (PRISm), a clinical entity as a prodromal phase of COPD. The aims of the present study were to evaluate the distributions of D(LCO) and to assess the association between D(LCO) and mortality among subjects with PRISm. PATIENTS AND METHODS: We conducted an observational cohort study at the National Hospital Organization Fukuoka National Hospital. We classified the 899 patients ≥ 40 years of age with an assessment of D(LCO) into five groups based on spirometry: preserved spirometry, PRISm, mild COPD, moderate COPD, and severe/very severe COPD. The prevalence of low D(LCO) (< 80% per predicted) was compared among the five groups. Using PRISm patients with follow-up data, we further investigated the association of low D(LCO) with all-cause mortality. RESULTS: The prevalence of low D(LCO) in the PRISm group (58.8%) was significantly higher than that in the preserved-spirometry group (21.8%), the mild-COPD group (23.5%), and the moderate-COPD group (36.0%) (all P < 0.01), and it was comparable to that in the severe/very severe-COPD group (63.2%). The results remained unchanged after adjusting for potential confounders. Among the PRISm subjects, the overall survival rate was significantly lower in the low-D(LCO) group than in the preserved-D(LCO) group (P < 0.01). The multivariable-adjusted hazard ratio (HR) for all-cause mortality was significantly higher in the low-D(LCO) group than in the preserved-D(LCO) group (HR = 10.10 (95% confidence interval 2.33–43.89)). CONCLUSION: Diffusing capacity was more impaired in PRISm subjects than in those with preserved spirometry or mild to moderate COPD. Regarding PRISm, low D(LCO) was a significant risk factor for all-cause mortality. Clinicians should assess D(LCO) in the management of PRISm to predict the future risk of overall death.