In the first 100 years following the design of Hutchinson?s spirometer, most research was focused on vital capacity. In 2014, Wan et al. introduced the concept of preserved ratio impaired spirometry (PRISm) presumably as a method to improve the poor predictive ability of FVC to identify true lung restriction, as represented by reduced total lung capacity (TLC). The definition of PRISm as a product of 2 binary variables (FEV1 %GLI or lower limit of normal (LLN)) increases the risk of error. This analysis aims to compare FVC and lung volumes in these PRISm definitions and to determine the prediction of true restriction.

PRISm (pre-BD FEV1<80%GLI or <LLN and FEV1/FVC?70%) was identified in the LEAD study, a single-centered, observational, population-based cohort study aged 6-82 years from Austria. The study population included 11060 subjects with valid lung function, including body plethysmographic measurement of lung volumes.

505 and 423 participants were found to have PRISm according to %GLI and LLN definitions, respectively: pre-BD FVC (%GLI) was 77.7±7.7 and 76.1±7.9 (p<0.05), TLC (%GLI) 92.3±10.6 and 92.1±11.3, and the RV/TLC (ratio) 39.4±8.9 and 39.6±9.8, for %GLI and LLN definitions, respectively. FVC pre<LLN was observed in 46.9% and 56.5% of PRISm subjects by %GLI or LLN definitions, respectively. True restriction (TLC<LLN) was found in 11.6% and 11.9% of participants with PRISM defined by %GLI and LLN, respectively.

Slightly lower FVC values in the presence of quite similar prevalence of low FVC are found in PRISm defined by LLN versus the fixed ratio. Importantly, true restrictive lung function is similar but low in both definitions (~12%) and raises concerns for the use of using PRISm to define lung restriction.