Validation of the Glaucoma Quality of Life-15 Questionnaire in Serbian language

Abstract

Aim: To translate the Glaucoma Quality of Life-15 (GQL-15) to Serbian language and asses its validity and reliability in the population of Serbian patients.

Methods: The study included 177 glaucoma patients. Clinical parameters (visual acuity, mean defect and square root of loss variance of visual field) and socio-demographic data were collected. Patients were stratified according to the Nelson's glaucoma staging system as mild, moderate and advanced glaucoma. All patients filled out the GQL-15 and National Eye Institute Visual Function Questionnaire (NEI-VFQ 25). The GQL-15 was translated following the internationally-accepted methodology, and its psychometric properties were assessed by using classical test theory and Rasch analysis.

Results: The mean total score for the GQL-15 was 20.68±7.31. The Cronbach's alpha coefficient for the whole scale was 0.89 (central and near vision, α=0.24; peripheral vision, α=0.85; glare and dark adaptation, α=0.83). Factor analysis established 4 factors (70.3% of variance): two corresponding to the original factors and two new factors specific for the Serbian population. The GQL-15 score correlated positively with almost all clinical parameters and NEI-VFQ 25 proving good criterion validity. Correlation of the GQL-15 total score on test-retest confirmed appropriate scale reproducibility (ρ=0.96, P<0.001). The GQL-15 discriminated well advanced from mild and moderate glaucoma. In Rasch analysis we obtained adequate item (0.95) reliability index. Almost all items had infit and outfit mean squares in the accepted range.

Conclusion: Serbian version of the GQL-15 demonstrates adequate reliability and validity. This version of the GQL-15 is a valid instrument for evaluation of quality of life among Serbian speaking patients with glaucoma and can be applied in daily clinical work.

Keywords: GQL-15; Serbian; glaucoma; scale validation.

Figures

Figure 1. Rasch analysis GQL-15 person item chart.