The minimal important difference of the King's Brief Interstitial Lung Disease Questionnaire (K-BILD) and forced vital capacity in interstitial lung disease.

Health status and forced vital capacity (FVC) are widely used outcome measures of interstitial lung disease (ILD) but there is a paucity of studies reporting the minimal clinically meaningful change in these parameters. A study was undertaken to assess the minimal important difference (MID) of an ILD specific health status questionnaire, the King's Brief ILD questionnaire (K-BILD) and that of FVC in a range of ILDs. 57 patients with ILD (17 idiopathic pulmonary fibrosis; IPF) completed the K-BILD (score range 0-100) at 2 separate clinic visits. Patients underwent spirometry at both visits. The MID was determined by a range of distribution methods (Standard error of mean: SEM and 0.3× Effect size: ES) and anchor based methods (objective: FVC and subjective: global rating of change questionnaires, GRCQ). The MID was derived by calculating an average of all methods. Health status was reduced at baseline in all patients, mean (SD) K-BILD total score 62(23). The average MID for K-BILD total score incorporating all methods was an 8 unit change (range 6-10). The average MID for FVC was a 6% change of baseline (range 4-7%). The K-BILD is a responsive patient reported outcome measure for patients with ILD. It can potentially be used to assess patients in the clinic and evaluate the response to therapy. The MID of the K-BILD total score is 8 units. The MID for FVC for a range of ILDs was 6%, similar to that reported recently for patients with IPF. Our findings will facilitate the clinical interpretation of health status and FVC data in ILD.