Original ArticleChanges to the subscales of two vision-related quality of life questionnaires are proposed
Introduction
During the last decades there has been increased attention for health-related quality of life (HRQOL) as an outcome measure in health-care [1], [2], [3]. The advantage over outcome measures at the impairment level is that HRQOL includes aspects of perceived health from a patient's perspective and the impact of health on a patient's life. The use of HRQOL as an outcome measure is especially appealing for use in populations with chronic conditions for which no cure exists. In these conditions, outcome measurements at the impairment level will show no or little improvement; however, improvement in HRQOL may occur through psychological adjustment, rehabilitation, or use of assistive devices.
The leading cause of severe visual impairment in the Western world is age-related macular degeneration (AMD). Other important causes are diabetic retinopathy (DRP) and glaucoma [4], [5], [6]. These are all chronic conditions for which no cure currently exists. Treatment is aimed at slowing down progression of the disease. For persons with severe visual impairment or blindness, rehabilitation programs exist that are aimed at helping them cope with their disability in daily life.
Not surprisingly, many quality of life (QOL) questionnaires have been developed specifically aimed at visually impaired people. In a recent review, as many as 31 such questionnaires have been identified [7]. Large differences in psychometric quality were found, and several areas of psychometric testing have been largely or totally neglected. One of these areas is the assessment of dimensions within a scale. Whereas a number of questionnaires were examined for factorial validity, none of these factor solutions have been confirmed in later studies, even when questionnaires were evaluated for different populations or in different languages [7]. A factor structure of a questionnaire that has been evaluated only once in one population cannot be assumed to be stable [8].
Other areas that need attention are the agreement between repeated measurements in reproducibility studies and the ascertainment of the minimal important change (MIC), which is also referred to as the minimal important difference [7]. Agreement measures refer to the absolute measurement error of an instrument, expressed in the unit of measurement of the instrument. Two well-known agreement measures are the 95% limits of agreement according to Bland and Altman [9] and the smallest detectable change (SDC), based on the standard error of measurement (SEM) [10].
The MIC can be defined as “the smallest difference in score in the domain of interest which patients perceive as beneficial and which would mandate, in the absence of troublesome side effects, a change in the patient's management” [2]. Assessment of a MIC is therefore very important for the interpretation of change scores in the target population. A logical next step is to link the MIC with the SDC, to assess whether the smallest difference perceived as beneficial from a patients' perspective can actually be detected beyond measurement error by the measurement instrument. Surprisingly, only a few authors have previously taken this final step in the evaluation of measurement instruments [11], [12].
Our main objective was to validate two vision-related QOL questionnaires in a Dutch population of persons with severe visual impairment (or low vision). Specific aims were (1) to confirm the factor structures of these questionnaires, (2) to assess the level of agreement between repeated measures by calculating the SDC, and (3) to assess the MIC and to examine if these questionnaires can be used to detect the MIC on individual and on group level.
Section snippets
Design
The study population consisted of participants of a 1-year follow-up study on the outcome of low vision services on the quality of life of visually impaired older men and women. Measurements were taken at baseline, 5 months later, and 1 year later. Confirmatory factor analyses were performed on the baseline data. The baseline and 5-month follow-up data were used to assess the MIC.
Participants who returned the baseline questionnaire and who had not yet had their appointment with low vision
Factor analyses
The questionnaires were completed by 329 participants at baseline. Mean age was 78.2 years (SD = 9.0); 62% of participants were female. The median Snellen visual acuity was 0.28 (interquartile range = 0.16–0.38). The main causes of visual impairment as recorded from the best eye were AMD (52%), DRP (13%), glaucoma (6%), corneal disease (6%), cataract (6%), and occluded vein (5%). Total percentage of missing values was 5.2% for the LVQOL and 0.2% for the VCM1. Of the 329 participants, 18 had
Factor analysis
The results show that the original factor structures of the VCM1 and the LVQOL could not be confirmed in our data set. The one factor structure of the original VCM1 is conceptually clear, except for the item on inability to do preferred activities. This item is clearly different from the other items, which are aimed at assessing feelings and perceptions associated with people's visual impairment. When the item was removed, the fit of the model increased considerably to a value > 0.9. The factor
Conclusion
In conclusion, we have revised the factor structure of two vision-related quality of life questionnaires using confirmatory factor analyses. The revised scales and subscales were then used to examine the smallest detectable change beyond measurement error for individuals and for groups and the minimal important change. The MIC was larger than the SDCind for all scales, meaning both questionnaires are not very useful in the follow-up of individual older visually impaired persons. There were
Acknowledgments
We would like to thank Herman Ader for his statistical advice concerning confirmatory factor analysis. Furthermore, we would like to thank the following Dutch foundations for financial support of this study: the ‘Landelijke Stichting voor Blinden en Slechtzienden’ (Michiel de Boer is LSBS-fellow), the ‘Stichting Blindenhulp’, the ‘Haak-Bastiaanse Kuneman Stichting’, and the ‘Stichting voor Ooglijders’.
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