How to classify the severity of a restriction?

<bold>Introduction:</bold> Total body plethysmography is the gold standard for confirming and classifying a restriction. While the 1991-ATS/ERS guidelines used total lung capacity (TLC) to classify the severity of a restriction the 2005-ATS/ERS guidelines shifted to forced expiratory volume in one second (FEV1). The 2022-ATS/ERS guidelines left it unclear which parameter should be used. <bold>Aim:</bold> To compare these two methods. <bold>Methods:</bold> This retrospective study included 93 patients referred to the Respiratory Functional Explorations Department at Farhat Hached University Hospital, Sousse. All patients underwent total body plethysmography with measurement of FEV1 and TLC. Z-scores were calculated according to GLI-2012 and GLI-2021 norms. Restriction was defined by a TLC’s z-score below "-1.64". Restriction severity was classified using two methods: M1 (TLC z-score) and M2 (FEV1 z-score). <bold>Results:</bold> Mean age was 55.14±12.36 years, with a sex ratio of 1.32. Of these patients, 41.9% were restrictive. Mean FEV1 and TLC were 80.21±19.38% and 84.91±18.41% respectively. Comparison of the two methods showed that 44.7% of patients would be unclassifiable using M2 (i.e., FEV1 z-score>-1.64). A significant difference was found in terms of severity of restriction, the percentage of patients with mild, moderate and severe restriction were respectively (M1: 13% vs M2: 23.7%), (M1: 55.3% vs M2: 28.9%) and (M1:13.2% vs M2:2.6%). <bold>Conclusion:</bold> The severity of the restriction is contingent upon the chosen definition. The poor agreement between classification based on CPT and that based on FEV1 poses challenges for clinicians, highlighting the need for standardized criteria to enhance diagnostic clarity and treatment planning.