Validation of a derived version of the IPF-specific Saint George’s Respiratory Questionnaire

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.