The association between transfer coefficient of the lung and the risk of exacerbation in asthma-COPD overlap: an observational cohort study

BACKGROUND: Asthma–chronic obstructive pulmonary disease (COPD) overlap (ACO) patients experience exacerbations more frequently than those with asthma or COPD alone. Since low diffusing capacity of the lung for carbon monoxide (D(LCO)) is known as a strong risk factor for severe exacerbation in COPD, D(LCO) or a transfer coefficient of the lung for carbon monoxide (K(CO)) is speculated to also be associated with the risk of exacerbations in ACO. METHODS: This study was conducted as an observational cohort survey at the National Hospital Organization Fukuoka National Hospital. D(LCO) and K(CO) were measured in 94 patients aged ≥ 40 years with a confirmed diagnosis of ACO. Multivariable-adjusted hazard ratios (HRs) for the exacerbation-free rate over one year were estimated and compared across the levels of D(LCO) and K(CO). RESULTS: Within one year, 33.3% of the cohort experienced exacerbations. After adjustment for potential confounders, low K(CO) (< 80% per predicted) was positively associated with the incidence of exacerbation (multivariable-adjusted HR = 3.71 (95% confidence interval 1.32–10.4)). The association between low D(LCO) (< 80% per predicted) and exacerbations showed similar trends, although it failed to reach statistical significance (multivariable-adjusted HR = 1.31 (95% confidence interval 0.55–3.11)). CONCLUSIONS: Low K(CO) was a significant risk factor for exacerbations among patients with ACO. Clinicians should be aware that ACO patients with impaired K(CO) are at increased risk of exacerbations and that careful management in such a population is mandatory. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12890-021-01815-w.