Reproducibility of pulmonary function tests in patients with systemic sclerosis

Systemic sclerosis (SSc) is a rare autoimmune disease in which interstitial lung disease (ILD) is the leading cause of morbidity and mortality. Clinical management of the lung disease is mainly based on pulmonary function testing (PFT) and their changes over time. Little is known about the reproducibility of PFT testing in SSc patients. The aim of this study was to assess the test-retest reliability and reproducibility of PFTs in SSc patients with or without ILD over 30 days in order determine the potential physiologic variation over the time. We performed prospective observational study of SSc patients. The FVC, FEV1/FVC ratio, DLCO and KCO parameters were assessed in this population at four different timepoints; T0 (time 0) and H3 (T0 + 3 h) defined test-retest reliability, D15 (T0 + 15 days) and D30 (T0 + 30 days) for reproducibility. A mixed linear model was used to test the effect of time (and therefore reproducibility) on patients and we looked for an interaction. We included 25 SSc patients divided in two groups, 14 with ILD and 11 non-ILD. Interactions between time and group were not significant and were not reported. Time and group did not significantly influence the different measures of the PFT: FVC [p values time and group effect respectively (0.33; 0.34)], FEV1/FVC ratio (0.093; 0.056) and DLCO (0.99; 0.13) in the ILD and non ILD group (Table S2). The analyse with interactions between time and group were not significant and are not reported. We also used a Bland Altman test to assess reproducibility for FVC (L) and DLCO (mMKpa/min/L), Figs. 1 and 2 respectively. The measurements were therefore reproducible over time and in each group. PFT parameters are reproducible over time in a clinically stable population of SSc (no significant effect of the time T0, H3, D15 and D30) and there is no significant distinction between patients with ILD and no ILD. These respiratory functional data can further underline their use in clinical practice.