Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G: reliability, construct validity, and agreement in cancer survivors

Health-related quality of life (HRQoL) is a critical outcome measure in oncology research and clinical care. This study aimed to translate, culturally adapt, and psychometrically validate the Bulgarian versions of the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire–Core 30 (EORTC QLQ-C30) and the Functional Assessment of Cancer Therapy–General (FACT-G), focusing on Internal Consistency (IC), Test–Retest Reliability (TRR), and Construct Validity (CV) in Cancer Survivors (CS) diagnosed with breast, lung, or colorectal cancer. A total of 235 Bulgarian CS (mean age 62.3 ± 11.2 years; 118 males, 117 females) were recruited from five tertiary care hospitals in a cross-sectional study design. The EORTC QLQ-C30 and FACT-G were translated and culturally adapted according to the official EORTC and FACIT guidelines. Psychometric evaluation included data quality (floor and ceiling effects, missing responses), IC, TRR, and CV assessed via convergent and Discriminant Validity (DV). A subsample of 78 study participants (33.2%) completed both questionnaires twice over a 14-day interval for TRR assessment. CV was supported by strong item-scale and cross-instrument correlations (r = 0.62–0.81). IC was excellent across all domains (Cronbach’s α = 0.88–0.95), and TRR demonstrated high stability (ICC = 0.75–0.89). Bland–Altman analysis showed acceptable agreement, with a mean difference of 23.20 (95% CI: 21.87–24.53) and limits of agreement ranging from +3.77 to +42.63. Minimal Floor Effects (FE) were observed, while Ceiling Effects (CE) were noted in specific domains (e.g., Social Functioning (SF) and Global Health (GH) in the EORTC QLQ-C30; Social Well-Being (SWB) and Emotional Well-Being (EWB) in the FACT-G). The findings support that the Bulgarian versions of the EORTC QLQ-C30 and FACT-G are valid, reliable, and culturally appropriate instruments for evaluating HRQoL in Bulgarian CS. Their use is recommended in both clinical practice and research contexts.