Changes to the subscales of two vision-related quality of life questionnaires are proposed
de Boer, Michiel R.; de Vet, Henrica C. W.; Terwee, Caroline B.; Moll, Annette C.; Volker-Dieben, Hennie J. M.; van Rens, Ger H. M. B.
Journal of Clinical Epidemiology
BACKGROUND AND OBJECTIVE: Psychometrically sound questionnaires for the assessment of vision-related quality of life (QOL) are scarce. Therefore, the objective was to further validate two vision-related QOL questionnaires in a Dutch population of visually impaired elderly. METHODS: A total of 329 visually impaired older persons referred to low vision services completed the low vision QOL (LVQOL) and Vision-Related Quality of Life Core Measure (VCM1) questionnaires at baseline, after 1-4 weeks (retest), and after 5 months. Confirmatory factor analyses were performed on baseline data. The smallest detectable change (SDC) was assessed, based on the standard error of measurement (SEM). Change scores between the baseline and 5 months follow-up data were related to a general transition question to assess the minimal important change (MIC). Furthermore, the MIC was related to the SDC, to examine whether the MICs were detectable beyond measurement error. RESULTS: The original factor structures could not be confirmed. After omitting items and remodeling, adequate fits were obtained. SDCs comprised at least one quarter of the scale for all scales and subscales on the individual level and exceeded the MICs on every occasion. CONCLUSION: We propose MICs of 5-10 points for the scales and subscales of the LVQOL and VCM1. The questionnaires are not useful in the follow-up of individual patients.