Referential equations for pulmonary diffusing capacity using GAMLSS models derived from Japanese individuals with near-normal lung function

Objective: To generate appropriate reference values for the single-breath diffusing capacity of the lungs for carbon monoxide (DLCO), alveolar volume (VA), and the transfer coefficient of the lungs for carbon monoxide (KCO, often denoted as DLCO/VA) in the Japanese population. We also intended to assess the applicability of these values for the Japanese population by comparing them to those published by the Global Lung Function Initiative in 2017 (GLI-2017) and previous values. Methods: In this retrospective study, we measured the spirometric indices, DLCO, VA, and KCO of the Japanese population aged 16–85 years. The lambda, mu, and sigma (LMS) method and the generalized additive models for the location, scale, and shape program in R were used to generate the reference values. Results: We conducted a total of 390 tests. The GLI-2017 z-scores of DLCO were approximately zero, whereas those of KCO and VA were far from zero. In the present study, the mean square errors of the DLCO, VA, and KCO reference values were lower than the reference values derived from GLI-2017 and previous linear regression equations. Conclusions: Reference values obtained in this study were more appropriate for our sample than those reported in GLI-2017. Differences between the two equations were attributed to underestimating KCO (DLCO / VA) and overestimating VA, respectively, by the GLI-2017 for the Japanese population.