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L. M. Niziol, N. K. Janz, D. C. Musch, B. W. Gillespie; Minimally Important Differences in Vision Specific Quality of Life Measures: Information From the CIGTS. Invest. Ophthalmol. Vis. Sci. 2010;51(13):951.
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To determine the minimally important differences (MIDs) for the Visual Activities Questionnaire (VAQ) and the National Eye Institute-Visual Function Questionnaire (NEI-VFQ).
607 subjects with newly-diagnosed open-angle glaucoma (OAG) were enrolled in the Collaborative Initial Glaucoma Treatment Study (CIGTS) between October, 1993 and April, 1997. Before randomization to initial treatment with medications or surgery, and every six months thereafter, subjects underwent an ophthalmic examination and telephone-administered quality of life (QOL) interview. The VAQ and NEI-VFQ were used to assess participants' perceptions of their visual function. Clinical measures included the mean deviation (MD) from Humphrey 24-2 full threshold visual field (VF) testing, and best-corrected visual acuity (VA) measured using the ETDRS protocol. Anchor- and distribution-based methods were used to develop MID ranges.
Anchor-based cross-sectional analyses at 66 months follow-up found a 10 letter increment of better eye VA corresponded to MIDs of 5.2 (1.1) and 3.8 (0.5) units on the VAQ and NEI-VFQ total scores, respectively. A 3 dB increment in the better eye MD yielded MIDs of 2.6 (0.8) and 2.3 (0.3) units for the same two questionnaires. In longitudinal analyses, we found MIDs for the VAQ of 3.2 (0.7) units for a 10 letter change of VA and 3.4 (0.6) units for a 3 dB change in the MD. A patient-reported global anchor on "how much glaucoma is interfering with QOL" yielded MIDs in a comparable range. Distribution-based MIDs were somewhat larger. MID values for the VAQ and NEI-VFQ subscales were also produced.
Based on extended follow-up data from a clinical trial of OAG treatment, we determined MIDs for two vision-specific QOL measures by employing multiple approaches to MID assessment. A range of MIDs for the VAQ (2.1 to 6.5 units) and NEI-VFQ (2.3 to 3.8 units) was found. Confidence in a specific MID value or range of values requires further confirmation by multiple studies.
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