Background: Health-related quality of life (HRQL) is impaired in patients with idiopathic pulmonary fibrosis (IPF). The King's Brief Interstitial Lung Disease questionnaire (K-BILD) is a validated measure of HRQL, but no previous studies have focused on the validity of K-BILD in IPF. Moreover, the relationship between K-BILD and dyspnoea or the 6-min walk test (6MWT) has not been assessed. The aim of this study was to validate K-BILD in the largest cohort of patients with IPF to date and assess how K-BILD correlates to dyspnoea and 6MWT.
Methods: Firstly, K-BILD was translated into Danish using validated translation procedures. Consecutive patients with IPF were recruited. At baseline, patients completed K-BILD, the IPF-specific version of St. Georges Respiratory Questionnaire, University of California, San Diego Shortness of Breath Questionnaire (SOBQ) Short Form-36, and pulmonary function tests and 6MWT were performed. After 14 days, K-BILD and Global Rating of Change Scales were completed. Internal consistency, concurrent validity, test-retest reliability and known groups validity were assessed. Analyses were also performed in subgroups of patients with different time since diagnosis.
Results: At baseline, 150 patients with IPF completed the questionnaires, and 139 patients completed the questionnaires after 14 days. K-BILD had a high internal consistency (Cronbach's α = 0.92). The concurrent validity was strong compared to SOBQ (r = - 0.66) and moderate compared to 6MWT (r = 0.43). Intraclass correlation coefficients (ICC = 0.91) and a Bland Altman plot demonstrated a good reliability. K-BILD was also able to discriminate between patients with different stages of disease (p < 0.002, Δscore > 7.4) and most results were comparable in patients with different time since diagnosis.
Conclusion: K-BILD is a valid and reliable instrument in patients with IPF and in patients with different time since diagnosis. To a major extent, K-BILD scores reflected the impact of dyspnoea on HRQL and the impact of physical functional capacity measured by the 6MWT to a moderate degree. Compared to PFTs alone, K-BILD provides additional information on the burden of living with IPF, and importantly, K-BILD is simple to implement in both research and clinical contexts.
Trial registration: Clinicaltrials.org (NCT02818712) on 30 June 2016.
Keywords: 6-min walk test; 6MWT; Danish; Health-related quality of life; IPF; Idiopathic pulmonary fibrosis; K-BILD; Quality of life; The King’s Brief Interstitial Lung Disease questionnaire; The St. Georges respiratory questionnaire.