Introduction. A restrictive ventilatory defect (RVD) is confirmed on the basis of a decrease in total lung capacity (TLC). According to the ATS/ERS 2021, the presence of restrictive impairment in an obstructed patient cannot be deduced from forced vital capacity (FVC) simple measurement. Aim.To determine the frequencies of obstructed patients with DVR according to the following definitions: ATS/ERS (2005): decreased TLC < 5th percentile, ATS/ERS (2021): isolated decrease in FVC below the lower limit of normal.

Methods. This was a cross-sectional study over a period of seven months. It included 300 Tunisian patients who underwent total body plethysmography. The GLI 2012 norms were applied for bronchial flow interpretation and the GLI 2021 norms for static lung volume interpretation. Statistical analysis was performed using SPSS 25 software.

Results. Only 40 patients were retained for the study. Median age was 57.5 [10 ; 82] years. Twenty-eight patients were male (70%) and 17 (42.5%) were overweight or obese. Among 300 patients who underwent total body plethysmography, 40 had an obstructive ventilatory defect (OVD) associated with a decreased FVC z-score, potentially reflecting a mixed ventilatory impairment. A reinterpretation of these results was performed, referring to the TLC z-score. Data expressed as Means ±standard deviations or medians of absolute values, and z-scores, were respectively,1.33±0.5 L, -3.77±0.82 for the forced expiratory volume in the first second (FEV1), 2.43±0.75 L, -2.38 for FVC and 5.8±1.98 L, -0.02±1.9 for TLC. The frequency of confirmed RVD was 17.5% (TLC z-score).

Conclusion. Isolated FVC decrease in OVD markedly overestimates the diagnosis of RVD.