Spirometric Interpretation and Clinical Relevance According to Different Reference Equations

Global Lung Function Initiative (GLI)-2012 reference equation is currently suggested for interpretation of spirometry results and a new local reference equation has been developed in South Korea. However, lung function profiles according to the different reference equations and their clinical relevance have not been identified in chronic obstructive pulmonary disease (COPD) patients.

Our cross-sectional study evaluated Choi's, Korean National Health and National Examination Survey (KNHANES)-VI, and GLI-2012 reference equations. We estimated the percentages of predictive forced expiratory volume in one second (FEV₁) and airflow limitation severity according to reference equations and analyzed their associations with patient reported outcomes (PROs): COPD assessment test (CAT) score, St. George's Respiratory Questionnaire for COPD patients (SGRQ-C) score, and six minute walk distance (6MWD).

In the eligible 2,180 COPD patients, lower predicted values of FEV₁ and forced vital capacity (FVC) were found in GLI-2012 compared to Choi's and KNHANES-VI equations. GLI-2012 equation resulted in a lower proportion of patients being classified as FEV₁ < 80% or FVC < 80% compared to the other equations. However, the Z-scores of FEV₁ and FVC were similar between the KNHANES-VI and GLI-2012 equations. Three reference equations exhibited significant associations between FEV₁ (%) and patient-reported outcomes (CAT score, SGRQ-C score, and 6MWD).

GLI-2012 reference equation may not accurately reflect FEV₁ (%) in the Korean population, but the Z-score using GLI-2012 equation can be a viable option for assessing FEV₁ and airflow limitation in COPD patients. Similar to the other two equations, the GLI-2012 equation demonstrated significant associations with PROs.