Abstract
BACKGROUND: Health-related quality of life (HRQL) is impaired in patients with idiopathic pulmonary fibrosis (IPF). HRQL is often measured using the St. George's Respiratory Questionnaire (SGRQ) despite the development of an IPF-specific version (SGRQ-I). Using data from a real-world cohort of patients with IPF, we aimed to transform SGRQ into a derived version of SGRQ-I, SGRQ-I(der), to examine the cross-sectional and longitudinal validity of SGRQ-I(der) and to compare SGRQ-I(der) to SGRQ-I. METHODS: Based on results from SGRQ, SGRQ-I(der) was derived applying the algorithm used to develop SGRQ-I. Of the 50 items in SGRQ, 34 items were retained in SGRQ-I(der). Response options for seven items were collapsed and minor adjustments were made to the weights of two items after correspondence with the developers of SGRQ-I. Cross-sectional validation, responsiveness and minimal clinically important difference (MCID) were assessed by comparison to other HRQL instruments, pulmonary function tests and 6-min walk test performed at baseline, 6 and 12 months. Furthermore, the association between SGRQ-I(der) scores and mortality was examined. RESULTS: A total of 150 IPF patients participated and 124 completed follow-up at 12 months. SGRQ-I(der) performed comparably to SGRQ-I with a high concurrent validity, good test-retest reliability and high known-groups validity. SGRQ-I(der) was responsive to change in HRQL and physiological anchors. MCID of SGRQ-I(der) for improvement and deterioration was 3.5 and 5.7, respectively. SGRQ-I(der) scores were associated with mortality in both univariate (HR 1.82, 95% CI 1.42-2.34 per 20-point increase) and multivariate analyses (HR 1.57, 95% CI 1.20-2.05 per 20-point increase). CONCLUSIONS: The SGRQ-I(der) is a valid, reliable and responsive HRQL instrument in patients with IPF and has psychometric properties comparable to SGRQ-I. Thus, SGRQ results can reliably be transformed into the SGRQ-I(der). The MCID estimates were calculated for improvement and deterioration separately. Increasing SGRQ-I(der) score was associated with increased mortality.