Estimating minimally important differences for two vision-specific quality of life measures

Abstract

Purpose: To estimate minimally important differences (MIDs) for the Visual Activities Questionnaire (VAQ) and the National Eye Institute-Visual Function Questionnaire (NEI-VFQ).

Methods: A total of 607 subjects with newly-diagnosed open-angle glaucoma (OAG) was enrolled in the Collaborative Initial Glaucoma Treatment Study (CIGTS) and randomized to initial treatment with medications or surgery. Subjects underwent an ophthalmic examination and telephone-administered quality of life (QOL) interview before randomization and every six months thereafter. 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 Early Treatment Diabetic Retinopathy Study (ETDRS) protocol. Anchor-based (using MD and VA) and distribution-based methods were used to estimate MIDs.

Results: Anchor-based cross-sectional analyses at 66 months follow-up found a 10-letter increment in better eye VA corresponded to MIDs of 5.2 units for VAQ and 3.8 units for NEI-VFQ total scores. A 3-dB increment in the better eye MD yielded MIDs of 2.6 and 2.3 units for the same two questionnaires. In longitudinal analyses, MIDs for the VAQ were 3.2 units for a 10-letter change of VA and 3.4 units for a 3-dB change in the MD. Distribution-based MIDs were larger.

Conclusions: A range of MIDs for the VAQ (2.6-6.5 units) and NEI-VFQ (2.3-3.8 units) was found. Although similar in magnitude, MIDs were sensitive to the MID estimation method, the anchor chosen, and differences between questionnaires. (ClinicalTrials.gov number, NCT00000149.).

Keywords: CIGTS; MID; NEI-VFQ; VAQ; glaucoma.

Figure 1

Boxplots showing change in VAQ subscales from baseline to last follow-up stratified by categories of MD change. Medians in the last panel of boxplots (for the VAQ peripheral vision) are zero for all categories of MD change. VAQ, Visual Activities Questionnaire; MD, mean devaition; dB, decibels.

Figure 2

Comparison of the MIDs between anchors (MD and VA), for each quality of life measure (VAQ and NEI-VFQ) and method (cross-sectional [X-Sec], longitudinal [Long], and distributional [Dist]). MID, minimally important difference; MD, mean deviation; VA, visual acuity; VAQ, Visual Activities Questionnaire; NEI-VFQ, National Eye Institute-Visual Function Questionnaire.