Cross-Cultural Adaptation and Initial Psychometric Evaluation of the Kazakh-Language Sino-Nasal Outcome Test-22 (SNOT-22) in Patients with Chronic Rhinosinusitis with Nasal Polyps

Background/Objectives: Chronic rhinosinusitis with nasal polyps (CRSwNP) is associated with substantial impairment of the health-related quality of life. The Sino-Nasal Outcome Test-22 (SNOT-22) is one of the most widely used patient-reported outcome measures in chronic rhinosinusitis. To our knowledge, no published psychometric evaluation of a Kazakh-language SNOT-22 has previously been reported. The aim of this study was to perform the cross-cultural adaptation and evaluate selected measurement properties of the Kazakh-language SNOT-22 in adults with CRSwNP. Methods: This prospective single-center study included 151 participants: 101 patients with CRSwNP (vitamin D supplementation group, n = 50; standard-treatment group, n = 51) and 50 healthy volunteers. Both patient groups underwent endoscopic sinus surgery and received topical glucocorticosteroids; adjunctive vitamin D was administered only in the vitamin D supplementation group according to the prespecified protocol. Cross-cultural adaptation was conducted according to ISPOR recommendations. The selected measurement properties examined were internal consistency, known-group validity, responsiveness, and exploratory associations with Lund–Kennedy endoscopic scores. Test–retest reliability and structural validity were not assessed. Results: Baseline SNOT-22 scores were substantially higher in patients with CRSwNP than in healthy volunteers (p < 0.001), although the interpretation of this known-group comparison is limited by significant age and sex imbalance between groups. In the combined target CRSwNP sample with complete item-level data (n = 99), Cronbach’s alpha was 0.921. Subgroup coefficients were 0.901 in the vitamin D supplementation group and 0.934 in the standard-treatment group, while the coefficient in healthy volunteers was 0.891. After one month, SNOT-22 scores decreased substantially in both patient groups (both p < 0.001), supporting the instrument’s ability to detect change over time. Baseline correlations with Lund–Kennedy scores were very weak and non-significant; stronger correlations were observed only after treatment. Conclusions: The Kazakh-language SNOT-22 showed promising initial measurement properties, including high internal consistency and the ability to distinguish patients from healthy volunteers and to detect longitudinal score changes. However, the psychometric evaluation remains incomplete because test–retest reliability, measurement error, and structural validity were not assessed, and evidence for convergent validity was limited. Further validation in larger, demographically balanced samples is required.