As health budgets tighten globally, evaluating the cost-effectiveness of glaucoma services is vital; however, there is a lack of validated instruments that measure utility specific to glaucoma patients. We thus evaluated the validity of the Visual Function Questionnaire—Utility Index (VFQ-UI) as a measure of vision-related function and preference-based status in glaucoma.
Cross-sectional study of 141 volunteer patients over 40 years of age with moderate (n=64) or severe (n=36) glaucoma, and 41 controls. Sociodemographic information, visual acuity, and visual field test parameters were obtained. The VFQ-UI and Glaucoma Activity Limitation-9 patient-reported outcome tools were administered, and their psychometric properties explored using Rasch analysis. Criterion, convergent, and divergent validity of the VFQ-UI were also assessed.
Mean age was 69.7 (±10.8) years, with a mean better eye visual acuity (LogMAR) of 0.074 (±0.356) (approximate Snellen acuity of 20/24). Overall, the VFQ-UI had satisfactory fit to the Rasch model, however targeting and precision were suboptimal with a person separation index of 1.72 and person reliability coefficient of 0.75. VFQ-UI scores significantly increased as glaucoma severity worsened between controls (−4.54, ±1.15) and patients with moderate (−3.77, ±1.57) and severe glaucoma (−1.98, ±2.98), indicating good criterion validity (P<0.001). Strong correlation between VFQ-UI and Glaucoma Activity Limitation-9 scores (r=0.764, P<0.001) demonstrated good convergent validity. There was no significant correlation between VFQ-UI scores and age and sex (r=0.150 and 0.026, respectively), demonstrating good divergent validity.
The VFQ-UI shows potential to measure preference-based status in a cohort of glaucoma patients, with the means to calculate utility. Although the VFQ-UI displayed reasonable fit to the Rasch model overall and had good criterion, convergent and divergent validity, its lack of precision is a limitation. The difficulty of the VFQ-UI in discriminating among patients with varying glaucoma severity may be improved by including items specific to glaucomatous visual dysfunction.
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*Centre for Eye Research Australia, The Royal Victorian Eye and Ear Hospital, The University of Melbourne, Melbourne
‡University of Melbourne Department of Surgery, Royal Melbourne Hospital, Parkville, Vic.
§Discipline of Ophthalmology, The University of Sydney, Sydney, Australia
†Ophthalmology Department, Addenbrookes Hospital, Cambridge, UK
S.E.S. had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
All authors have made substantial contributions to the intellectual content of the paper in acquisition, analysis, or interpretation of data; drafting of the manuscript; and critical revision of the manuscript for important intellectual content.
E.F. is funded by a National Health and Medical Research Council Early Career Fellowship (#1072987). The Centre for Eye Research Australia receives Operational Infrastructure Support from the Victorian Government.
Disclosure: The authors declare no conflict of interest.
Reprints: Simon E. Skalicky, FRANZCO, MPhil, MMed (Ophthal Sci), MBBS (Hons 1), BSc (Med), 32 Gisborne St East Melbourne, Vic. 3002, Australia (e-mail: firstname.lastname@example.org).
Received January 28, 2016
Accepted April 19, 2016