Structural validity and measurement invariance of the WHO-5 Well-Being Index among young adolescents in a family-focused prevention context in Southeastern Europe

Background The World Health Organization Five Well-Being Index (WHO-5) is a widely used, open-access measure of subjective well-being. However, evidence on its psychometric properties among young adolescents participating in preventive interventions in low- and middle-income countries remains limited. This study aims to evaluate the structural validity and cross-country measurement invariance of the WHO-5 among young adolescents participating in a prevention program in Southeastern Europe. Methods Data were drawn from the baseline assessment of the FLOURISH Phase 2 trial, including 653 adolescents aged 10–14 years recruited with their caregivers as part of a family-focused program in Moldova and North Macedonia. The WHO-5 was administered using culturally adapted versions in local languages. Confirmatory factor analysis tested the one-factor, five-item model in the overall sample and separately in each country. Model fit was evaluated using the RMSEA, CFI, TLI and SRMR. Multi-group ordinal confirmatory factor analysis assessed configural, threshold, metric and scalar invariance across countries. Internal consistency was estimated using McDonald’s ω. Results The one-factor, five-item model showed appropriate fit in the overall sample, CFI = 0.995, TLI = 0.990, RMSEA = 0.069 and SRMR = 0.019. Country-specific analyses also supported the one-factor structure. Standardized factor loadings ranged from 0.698 to 0.811 and residual variances ranged from 0.343 to 0.513. McDonald’s ω was 0.84. Measurement invariance analyses supported configural, threshold, metric and scalar invariance. Conclusions The WHO-5 demonstrated good internal consistency, a unidimensional structure and cross-country measurement invariance among young adolescents participating in a prevention program in Moldova and North Macedonia. These findings support the use of the WHO-5 for assessing and comparing adolescent well-being across these settings and contribute to the validation of open-access, cross-culturally adapted instruments for well-being monitoring in middle-income and prevention contexts. Trial registration The parent FLOURISH Phase 2 trial was prospectively registered with ClinicalTrials.gov (NCT06562244). The published trial protocol is available at https://doi.org/10.1136/bmjopen-2024-094085 . Project page: https://www.flourish-study .