The development and validation of the King's Brief Interstitial Lung Disease (K-BILD) health status questionnaire.

RATIONALE Health status is impaired in patients with interstitial lung disease (ILD). There is a paucity of tools that assess health status in ILD. The objective of this study was to develop and validate the King's Brief Interstitial Lung Disease questionnaire (K-BILD), a new health status measure for patients with ILD. METHODS Patients with ILD were recruited from outpatient clinics. The development of the questionnaire consisted of three phases: item generation; item reduction, allocation to domains by factor analysis, Rasch analysis to create unidimensional scales and validation; and repeatability testing. RESULTS 173 patients with ILD (49 with idiopathic pulmonary fibrosis) completed a preliminary 71-item questionnaire. 56 items were removed due to redundancy, low factor loadings or poor fit to the Rasch model. The final version of the K-BILD questionnaire consisted of 15 items and three domains (breathlessness and activities, chest symptoms and psychological). Internal consistency assessed with Cronbach's α coefficient was 0.94 for the K-BILD total score. Concurrent validity of the K-BILD questionnaire was high compared with St George's Respiratory Questionnaire (r=0.90) and moderate with lung function (vital capacity, r=0.50). The K-BILD questionnaire was repeatable over 2 weeks (n=44), with intraclass correlation coefficients for domains and total score 0.86-0.94. The K-BILD construct validity for patients with idiopathic pulmonary fibrosis was similar to that of other ILDs. CONCLUSION The K-BILD questionnaire is a brief, valid, self-completed health status measure for ILD. It could be used in the clinic to assess ILD from the patients' perspective.