BackgroundFibrotic interstitial lung disease (ILD) is often associated with poor outcomes, but has few predictors of progression. Daily home spirometry has been proposed to provide important information about the clinical course of idiopathic pulmonary disease (IPF). However, experience is limited, and home spirometry is not a routine component of patient care in ILD. Using home spirometry, we aimed to investigate the predictive potential of daily measurements of forced vital capacity (FVC) in fibrotic ILD.MethodsIn this prospective observational study, patients with fibrotic ILD and clinical progression were provided with home spirometers for daily measurements over 6 months. Hospital based spirometry was performed after three and 6 months. Disease progression, defined as death, lung transplantation, acute exacerbation or FVC decline >10% relative was assessed in the cohort.ResultsFrom May 2017 until August 2018, we included 47 patients (IPF n=20; non-IPF n=27). Sufficient daily measurements were performed by 85.1% of the study cohort. Among these 40 patients (IPF n=17; non-IPF n=23), who had a meanSD age of 60.7 +/- 11.3years and FVC 64.7 +/- 21.7% predicted (2.4 +/- 0.8L), 12 patients experienced disease progression (death: n= 2; lung transplantation: n=3; acute exacerbation: n=1; FVC decline >10%: n=6). Within the first 28days, a group of patients had high daily variability in FVC, with 60.0% having a variation >= 5%. Patients with disease progression had significantly higher FVC variability than those in the stable group (median variability 8.6% vs. 4.8%; p=0.002). Cox regression identified FVC variability as independently associated with disease progression when controlling for multiple confounding variables (hazard ratio: 1.203; 95% CI:1.050-1.378; p=0.0076).Conclusions Daily home spirometry is feasible in IPF and non-IPF ILD and facilitates the identification of FVC variability, which was associated with disease progression.