Prevalence, longitudinal trajectory, and mortality of preserved ratio impaired spirometry (PRISm) in a Thai hospital-based cohort study.

BACKGROUND: Preserved ratio impaired spirometry (PRISm) is spirometric pattern characterized by reduced forced expiratory volume in 1 second (FEV1) despite a preserved FEV1/forced vital capacity (FVC) ratio. PRISm has been associated with respiratory symptoms, impaired lung function, and increased mortality; however, data from Southeast Asian populations remain limited. This study aimed to determine the prevalence, spirometric trajectory, contributing factors, and mortality of PRISm in a Thai hospital-based cohort. METHODS: We conducted a retrospective cohort study of adults who underwent spirometry at Siriraj Pulmonary Function Laboratory between April 2017 and February 2018, with follow-up through March 2024. PRISm was defined using either: (I) lower limit of normal (LLN) criteria: FEV1 < LLN, FVC ≥ LLN and FEV1/FVC ≥ LLN; or (II) fixed-ratio criteria: FEV1 <80% predicted, FVC ≥80% predicted and FEV1/FVC ≥75% for age ≤45 years or ≥70% for age >45 years. Demographic characteristics, co-morbidities, trajectories and mortality were analyzed. Logistic regression and survival analyses were performed. RESULTS: Among 1,300 subjects, the overall prevalence of PRISm was 8.4% when either definition was applied. The prevalence was 3.5% using LLN criteria, and 6.0% using fixed-ratio criteria. Compared with normal spirometry, those with PRISm were older age and had a higher prevalence of congestive heart failure. PRISm was associated with increased all-cause mortality compared with normal [hazard ratio (HR) 2.37, 95% confidence interval (CI): 1.19-4.72, P=0.001] and remained independently associated after adjustment (aHR 2.08, 95% CI: 1.04-4.15, P=0.04). Among 52 subjects with follow-up spirometry, 30.8% transitioned to normal, 28.9% developed obstructive impairment, 19.2% developed restrictive impairment, and 21.1% remained PRISm. CONCLUSIONS: PRISm is a clinically relevant spirometric pattern associated with increased mortality in this Thai cohort. Its dynamic trajectory highlights the importance of longitudinal monitoring and further research to clarify its natural history.