Home handheld spirometry enables repeated measurements of forced vital capacity (FVC), offering opportunities for longitudinal evaluation in interstitial lung disease (ILD). Whilst recent studies have not blinded participants to their home spirometry performance, they support feasibility in participants with idiopathic pulmonary fibrosis (IPF). However little data exists for the utility of home spirometry in non-IPF ILD. We assess correlation, agreement and non-inferiority of blinded daily home spirometry over three months relative to hospital spirometry, informing the feasibility of remote monitoring as a primary endpoint in clinical settings.
We utilised interim data from the ongoing INJUSTIS study (NCT03670576). Participants with fibrotic ILD were offered a handheld spirometer linked via bluetooth to a smartphone application and asked to perform daily, blinded FVC for three months. Hospital spirometry was concurrently obtained at baseline and three months. Home FVC values were based on week averages at study timepoints. Correlation, Bland-Altman plots and equivalence tests were used to compare baseline, 3 month and delta. Sensitivity analysis was performed where test dates matched.
82 participants with ILD were included. Mean age was 69.8±8 years, 72.3% were male and mean FVC was 2.96±0.88L. Median adherence to daily spirometry was 79.5%, four participants had an adherence <10%. At the time of censorship, 35 participants had 3 month data for both home and hospital spirometry, 45 participants had date-matched values. High correlation was observed between home and hospital spirometry at baseline (r=0.86) and three-months (r=0.81), changes in 3 month ΔFVC were not correlated (r=-0.09). At least 90% of home spirometry values were within agreement limits of hospital values at baseline (mean difference -0.31L/min,95%CI -0.39;-0.22), three-months (-0.13L/min,95%CI -0.31;0.05) and 3 month ΔFVC (-0.03L/min,95%CI -0.13;0.20). Home values more frequently underestimated hospital values but non-inferiority was confirmed within 400 ml.
Home spirometry in fibrotic ILD is feasible and non-inferior to hospital spirometry. This is particularly relevant in the context of the current covid-19 pandemic, where an urgent need has arisen to consider remote monitoring of lung function. Adherence to daily spirometry was high in blinded participants, but variability in home values was observed when using week-averages, supporting importance of longitudinal modelling for clinical endpoint precision.
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