Introduction.

Spirometry is the first line diagnostic test in front of respiratory symptoms and might avert lung volume measurement when it is normal. According to ATS/ERS latest guidelines (2022), it can usually exclude a restrictive ventilatory defect (RVD) when the forced vital capacity (FCV) is normal. However, it remains a controversial issue.

Objective.

To determine the accuracy with which a normal FVC can exclude a restrictive ventilatory defect.

Methods.

It was a retrospective study conducted in the department of physiology and functional explorations of the university hospital Farhat HACHED of Sousse. It included 96 patients aged 55.68 ±13.37 years (48.6% male) who underwent complete pulmonary function testing using Medisoft body box plethysmograph. FVC and total lung capacity (TLC) were determined, and their respective z-scores were calculated according to the GLI norms for spirometric data (2012) and static lung volumes (2021). A z-score greater than "-1.64" was considered normal.

Results.

Among the 96 patients included, 43 had a RVD (ie, z-score of TLC below " -1.64"). This RVD was mild, moderate, and severe in 51.2% (n=22), 34.9% (n=15) and in 14% (n=6) of the cases, respectively. The FVC was normal in 31.7% (n=13) of patients with a RVD. The sensitivity of the FVC in diagnosing a RVD was relatively low at 68.29%, the specificity at 78.18%, and the negative predictive value at 76.79%.

Conclusion.

The ATS/ERS statement that a normal FVC excludes a restriction is not applicable to Tunisian population. Therefore, a normal FVC should not discourage physicians from pursuing lung volumes measurement to rule out a RVD with certainty.