Psychometric properties of the Glaucoma Quality of Life-15 questionnaire: Use of explanatory factor analysis

The purposes of this study were to validate the Persian translation of the Glaucoma Quality of Life-15 (GQL-15) questionnaire, evaluate its psychometric properties, and identify new composite items and item numbers.
This cross-sectional study was conducted from August to November 2016, at the Glaucoma Clinic of the Ophthalmology Department at Shiraz University of Medical Sciences, Iran. One hundred ninety patients with glaucoma were enrolled. Habitual-corrected visual acuity (HCVA), intraocular pressure (IOP), slit-lamp biomicroscopy, fundus exam, and mean deviation (MD) of the visual field were recorded in the course of clinical examination by glaucoma professional. Psychometric properties, i.e. test–retest reliability, internal consistency, content validity, and construct validity were evaluated with factor analysis. Based on the Disc Damage Likelihood Scale (DDLS), patients were stratified to mild, moderate, and severe disc damage. The association between the GQL-15 scores and disease severity (mild, moderate and severe) were evaluated by the analysis of variance (ANOVA).
Of 190 eligible glaucoma patients, reliable clinical data were available for 140 participants. Mean age [standard deviation (SD)] of the patients was 58.7 (13.3) years. Cronbach's α coefficient ranged from 0.74 to 0.91, and the correlation coefficient for total score was 0.53. The content validity ratio (CVR) was 0.91 based on evaluations in expert panel. Exploratory factor analysis (EFA) based on eigenvalue higher than one identified two factors after varimax rotation for the GQL-15 which explained 66.5% of the total variance. Discriminant validity analysis disclosed statistically significant differences in mean quality of life scores between levels of disease severity.
The Persian version of the GQL-15 is a reliable and valid questionnaire for use in glaucoma clinics as a complementary tool for evidence-based decision-making.