Measuring low vision service outcomes: Rasch analysis of the seven-item National Eye Institute Visual Function...

To describe a short, functional visual disability instrument capable of measuring the outcomes of a government funded country-wide multicenter low vision service in Wales and to determine if postal implementation might lead to response bias.

Seven items from the National Eye Institute Visual Function Questionnaire (NEI VFQ), which have previously been shown to be responsive to low vision service intervention, were incorporated into a postal questionnaire which was given to patients before they attended the all Wales Low Vision Service. Rasch analysis was used to describe the instrument's psychometric properties.

Rasch analysis of 490 completed questionnaires showed that all seven questions worked together to form a unidimensional scale. By combining the first two response categories, category utilization and targeting was improved. There were no significant differences in the age (p = 0.29), gender (p = 0.75), distance binocular Visual Acuity (VA; p = 0.86), living situation (p = 0.34) or prevalence of macular degeneration (p = 0.31) between those who returned a completed questionnaire and those who did not.

The seven-item National Eye Institute Visual Function Questionnaire is an appropriate and precise outcome measure that is acceptable to patients and easy to administer. It measures aspects of near, reading, and distance visual disability that have been shown to be amenable to low vision service provision and, therefore, it should be highly responsive to this intervention and facilitate inter-service assessment. We found no evidence to suggest that postal implementation results in response bias. The instrument is an appropriate measure of patient based outcomes for a large-scale, multicenter low vision service.