Restrictive spirometry (RS) and Preserved Ratio Impaired Spirometry (PRISm) are conditions of low lung function characterized by proportionate reductions in FVC (RS) or FEV1 (PRISm) in the presence of a nonobstructive FEV1/FVC ratio (?70%). Controversy exists whether a fixed value (%GLI) or lower limit of normal thresholds (LLN) should be used to define these conditions.

The present study compares the impact of both cut-offs (RS: FVC%GLI or LLN; PRISm: FEV1%GLI or LLN) in the Austrian LEAD study, a single-centered, observational, population-based cohort study. Lung volumes (total lung capacity (TLC), RV/TLC, maximal mid-expiratory flow (MMEF) and specific airway conductance (sGAW) were compared between groups. The study population included 9967 subjects with valid lung function and normal TLC.

380 and 315 subjects fulfill the criteria of RS based on %GLI or LLN, respectively, and 74% of those with fixed cut-off of FVC are classified as RS, based on LLN.
452 and 376 subjects fulfill the criteria of PRISm based on %GLI or LLN, respectively, and 66% of those with fixed cut-off of FVC are classified as PRISm, based on LLN.

In both RS and PRISm, respective definitions resulted in similar RV/TLC (ratio; ~40±9.0% in both), TLC (%GLI; ~91±7.5% in RS, ~94±8.5% in PRISm), MMEF (%GLI; ~80±24% in RSP, ~62±14% in PRISm) and sGAW (1/sRAW; ~1.2±0.5 in RS, ~1.0±0.4 in PRISm).

Classification of conditions of low lung function in the presence of nonobstructive FEV1/FVC ratio (RS and PRISm) highly depend on cut-off criteria for both conditions. While lung volumes were similar in both definitions used, the impact of classification criteria needs to be considered in studies assessing the clinical spectrum of these conditions.