The SF-36 and SGRQ: Validity and first look at minimum important differences in IPF

Abstract

Health-related quality of life (HRQL) is an important outcome in drug trials. Little is known about how the Short Form-36 (SF-36) and Saint George's Respiratory Questionnaire (SGRQ) perform in idiopathic pulmonary fibrosis (IPF). To examine the validity of the SF-36 and SGRQ and to determine scores from each that would constitute a minimum important difference (MID). We analyzed data from a recently completed trial that enrolled subjects with well-defined IPF who completed the SF-36, SGRQ, and Baseline/Transition Dyspnea Index at baseline and six months. We compared mean changes in HRQL scores between groups of subjects whose disease severity changed over six months according to clinical anchors (FVC, DLCO, and dyspnea). We estimated the MID for each domain by using both anchor- and distribution-based approaches. Results supported the validity of the SF-36 and SGRQ for use in longitudinal studies. Mean changes in domain scores differed significantly between subjects whose clinical status improved and those whose clinical status declined according to the anchors. MID estimates for the SF-36 ranged from 2-4 points and from 5-8 points for the SGRQ. In IPF, the SF-36 and SGRQ possess reasonable validity for differentiating subjects whose disease severity changes over time. More studies are needed to continue the validation process, to refine estimates of the MIDs for the SF-36 or SGRQ, and to determine if a disease-specific instrument will perform better than either of these.