The psychometric properties of the King's Brief Interstitial Lung Disease questionnaire and thresholds for meaningful treatment response in patients with progressive fibrosing interstitial lung diseases

BACKGROUND: There is a lack of fully validated patient-reported outcome measures for progressive fibrosing interstitial lung disease (ILD). We aimed to validate the King's Brief Interstitial Lung Disease (K-BILD) questionnaire for measuring health-related quality of life (HRQoL) in these patients. We also aimed to estimate the meaningful change threshold for interpreting stabilisation of HRQoL as a clinical end-point in progressive fibrosing ILD, where the current goal of treatment is disease stability and slowing progression. METHODS: This analysis evaluated data from 663 patients with progressive fibrosing ILD other than idiopathic pulmonary fibrosis from the INBUILD trial. Validation of the measurement properties was assessed for internal consistency, test–retest reliability, construct validity, known-groups validity and responsiveness. We calculated meaningful change thresholds for treatment response using anchor-based (within-patient) and distribution-based methods. RESULTS: K-BILD had strong internal consistency (Cronbach's α was 0.94 for total score, 0.88 for breathlessness and activities, 0.91 for psychological, and 0.79 for chest symptoms). The test–retest reliability intraclass correlation coefficient was 0.74 for K-BILD total score. K-BILD demonstrated weak correlations with forced vital capacity (FVC) percent predicted. Known-groups validity showed significant differences in K-BILD scores for patient groups with different disease severity based on use of supplemental oxygen or baseline FVC % pred (≤70% or >70%). We estimated a meaningful change threshold of ≥ –2 units for K-BILD total score for defining patients who remain stable/improved versus those with progressive deterioration. CONCLUSIONS: Our results validate K-BILD as a tool for assessing HRQoL in patients with progressive fibrosing ILD and set a meaningful change threshold of ≥ –2 units for K-BILD total score.