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.

There is a lack of fully validated patient-reported outcome measures for progressive fibrosing interstitial lung disease (PF-ILD). We aimed to validate the King's Brief Interstitial Lung Disease questionnaire (KBILD) for measuring health-related quality of life (HRQoL) in these patients. We also estimated the meaningful change threshold for interpreting stabilisation of HRQoL as a clinical endpoint in PF-ILD, where the current goal of treatment is disease stability and slowing progression. This analysis evaluated data from 663 patients with PF-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. The KBILD had strong internal consistency (Cronbach alpha was 0.94 for total score, 0.88 for KBILD domain breathlessness and activities, 0.91 for psychological and 0.79 for chest symptoms). Test-retest reliability intraclass correlation coefficient was 0.74 for KBILD total score. The KBILD demonstrated weak correlations with forced vital capacity (FVC)% predicted. Known-groups validity showed significant differences in KBILD scores for patient groups with different disease severity based on use of supplemental oxygen or baseline FVC% predicted (<70 or >70%). We estimated a meaningful change threshold of ≥-2 for the KBILD total score for defining patients who remain stable/improved versus those with progressive deterioration. Our results validate the KBILD as a tool for assessing HRQoL in patients with PF-ILD and set a meaningful change threshold of ≥-2 for KBILD total score.