Self-assessed visual function outcome in cataract surgery: minimum important difference of the Catquest-9SF questionnaire

BACKGROUND: The purpose of this study was to study the minimum important difference (MID) of the Catquest-9SF questionnaire in cataract surgery. METHODS: A nationwide multi-center prospective randomized study was conducted using the Swedish National Cataract Register and the Catquest-9SF questionnaire. Randomized patients (n = 400) who had completed the Catquest-9SF before surgery and three months after surgery were sent an anchor question on self-assessed change in visual function after cataract surgery 14 days after the postoperative Catquest-9SF. Rasch analysis was performed on the preoperative and postoperative Catquest-9SF questionnaires, and the patients were dichotomized with regard to their preoperative Rasch score. The MID range of the two groups was calculated based on the anchor question, and the anchor question based MID was then estimated in a scatter plot. The MID was also estimated based on distribution by calculating Cohen’s effect size. RESULTS: The analyses included 231 patients who had completed the Catquest-9SF on both occasions as well as the questionnaire with the anchor question. The group with better preoperative visual function had an anchor question based MID of − 0.5 and a Cohen’s effect size based MID of − 1.07. The group with worse preoperative visual function had an anchor question based MID of − 1.80 and a Cohen’s effect size based MID of − 1.46. CONCLUSION: This article contributes detailed knowledge of the MID of Catquest-9SF, enabling even more accurate high-quality evaluation of the outcome and benefit of cataract surgery worldwide.