Zhang, Z.

Abstract Rationale Pulmonary function tests (PFT) are interpreted in the context of reference values that were based on sex, age, height and previously race/ethnicity. The Global Lung Function Initiative (GLI) published spirometry references equations in 2012 and static lung volume reference equations in 2021. In 2023, the American Thoracic Society recommended removal of race from predictive equations for spirometry using the new race-neutral GLI-Global reference equations. Since the change of spirometry reference equations, we have noticed an increase in number of patients with normal spirometry but a low total lung capacity (TLC), where historically the incidence was less than 5 percent and normal spirometry was used to exclude restriction. The goal of this study was to further investigate this observation and characterize the population of patients with normal spirometry and reduced TLC, which we termed occult restriction. Methods We analyzed our existing PFT database which included 28,415 spirometry tests performed at Mayo Clinic Rochester and Mayo Clinic Florida between 2013 to 2016. 26,856 tests included lung volume measurements. The spirometry was interpreted using both GLI-2012 and GLI-Global reference equations and TLC was interpreted using GLI lung volume reference equations. Patients’ demographics including self-identified race were obtained. Patients with occult restriction were further characterized using descriptive statistics. All data analysis was done using BlueSky Statistics Version 10.3.1. Results Our study showed that 920 (3.4%) patients with normal spirometry using GLI-2012 reference equations had low TLC, while 1526 (5.7%) patients with normal spirometry using GLI-Global reference equations had low TLC. Among these patients, 697 patients had a change from abnormal to normal spirometry interpretation and 91 patients had a change from normal to abnormal spirometry interpretation when using GLI-Global reference equations. Comparing patients of different races and ethnicities, there is increased incidence of occult restriction in Caucasian patients with transition to GLI-Global, while the incidence of occult restriction decreased in African American patients. Patients who identify as African American or Southeast Asian are more likely to have occult restriction than others (Table 1). Conclusions With the change from GLI-2012 to GLI-Global spirometry reference equations, the frequency of occult restriction has increased from 3.4% to 5.7% with predominant effect on Caucasian patients. The clinical implication of this finding requires further study but clinicians need to be vigilant that a normal spirometry may not exclude restrictive lung disease using our current reference equations.