Interpretation of change scores for the National Eye Institute Visual Function Questionnaire-25: the minimally important difference.

Aim: To estimate the minimally important difference (MID) in change in National Eye Institute Visual Function Questionnaire-25 (VFQ-25) composite score using methods aligned with patient perception.
Methods: Retrospective analysis of prospectively collected data from adults with primary angle closure or primary angle closure glaucoma enrolled in the Effectiveness, in Angle-closure Glaucoma, of Lens Extraction study. We included data from 335 participants with patient reported visual function (VFQ-25) and health status measured by the EQ-5D-3L over 36 months. We used the recommended anchor-based methods (receiver operating characteristic (ROC), predictive modelling and mean change) to determine the MID of the VFQ-25. EQ-5D-3L anchor change was defined as none (<0.065); minimal (0.065≤EQ-5D-3L change ≤0.075 points) and greater change (>0.075 points).
Results: Mean baseline VFQ-25 score was 87.6 (SD 11.8). Estimated MIDs in the change in VFQ-25 scores (95% CI) were 10.5 (1.9 to 19.2); 3.9 (-2.3 to 10.1); 5.8 (1.9 to 7.2) and 8.1 (1.7 to 14.8) for the 'within-patient', 'between-patient' change, ROC and predictive modelling anchor methods respectively. Excluding estimates from the methodologically weaker 'within-patient' method, the MID of a change in VFQ-25 composite score is 5.8 (median value).
Conclusions: Estimates of the MID using multiple methods assist in the interpretation of the VFQ scores. In the context of early glaucoma related visual disability, a change score of around six points on the VFQ-25 is likely to be important to patients. Further confirmatory research is required. Studies comparing changes in patient-reported outcome measure scores with a global measure of patients' perceived change are required.
Competing Interests: Competing interests: None declared.
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