Glaucoma Symptom Scale: Psychometric properties of the Serbian version

Purpose: The study aim was to translate and validate the Glaucoma Symptom Scale (GSS) in Serbian language. Methods: Clinical parameters and socio-demographic data were collected for each of the 177 enrolled glaucoma patients. Each eye was classified according to the Glaucoma staging system by Mills into 6 stages. Patients filled out the GSS and National Eye Institute Visual Function Questionnaire (NEI-VFQ 25). The GSS comprises 10 complaints common for glaucoma patients on a topical treatment, grouped into two subscales: SYMP-6 (non-visual) and FUNC-4 (visual problems). The GSS was translated following the customary methodology and its psychometric properties were assessed by using both Classical Test Theory (CTT) and Rasch analysis. Results: The internal consistency of the Serbian GSS for the whole scale was very good (Cronbach’s alpha = 0.81). On factor analysis items were clustered into 2 factors (48.92% of variance) which corresponded to the original scale. The total and subscale GSS scores correlated significantly with measures of disease severity and also with total score and analogous NEI-VFQ 25 subscale scores. In Rasch analysis we obtained adequate item reliability index (0.90). Almost all items had infit and outfit mean squares in the accepted range. However, measurement precision was poor (low person separation reliability) and targeting revealed a ceiling effect. Conclusion: When analyzed with CTT the Serbian version of the GSS seems to be a valid instrument, but Rasch analysis revealed some serious measurement flaws, therefore it should not be used in its current format. Further studies to modify and improve GSS are needed prior to its application for Serbian glaucoma patients.