Patient-reported quality of life for cataract surgery: prospective validation of the ‘Impact on Life’ and Catquest-9SF questionnaires in New Zealand

Abstract

AIMS: The 'Impact on Life' (IoL) questionnaire is used to prioritise publicly funded cataract surgery in New Zealand, however, it has not been formally validated for ophthalmic use. The Catquest-9SF questionnaire is widely used to assess vision-related quality of life (VRQoL) but has not been validated in New Zealand. This study evaluates the validity of the IoL and Catquest-9SF questionnaires for measuring VRQoL in New Zealand. METHOD: Formal ethics approval was obtained. Participants completed the IoL and Catquest-9SF questionnaires before and three months after routine cataract surgery. Rasch analysis was used to investigate all qualitative questionnaire responses. Results were correlated with the change in patient visual acuity. RESULTS: There was a 100% response rate at follow-up (41 participants). Disordered probability thresholds were observed for all IoL questions but no Catquest-9SF questions. All IoL questions demonstrated unsatisfactory mean-square fit statistics. Differences in visual acuity following surgery correlated with the change in total F-score for the Catquest-9SF (P=0.04), but not IoL responses (P=0.17). CONCLUSIONS: Disordered probability thresholds, poor question-model fit and correlation with visual acuity changes indicate the current IoL questionnaire is poorly suited for assessment of VRQoL. In contrast, the Catquest-9SF demonstrated credible results for assessment of VRQoL in New Zealand.