Validation of the COmprehensive Score for Financial Toxicity (COST) in Vietnamese patients with cancer.

Introduction: The COmprehensive Score for Financial Toxicity (COST) has proven to be a reliable tool for quantifying the impact of financial toxicity (FT) in patients with cancer in clinical and public health settings. However, the COST has not yet been validated in Vietnam. Therefore, we aimed to evaluate its reliability and validity among Vietnamese patients with cancer.

Methods: A cross-sectional study was conducted in a sample of 300 patients with cancer aged 27-95 years (mean: 58.5±11.2) in a tertiary hospital. The COST was translated into Vietnamese and English and adjusted to suit the local culture. Reliability was evaluated using Cronbach's alpha and McDonald's omega coefficients. The construct and convergent validities were also assessed.

Results: The COST demonstrated good internal consistency and reliability (Cronbach's alpha = 0.913; McDonald's omega = 0.915). The exploratory factor analysis revealed two factors that explained 64.9% of the variance. The adjusted fit indices indicated a good fit of the model (χ2 (39) = 67.78, p = 0.003; standardized root mean squared residual = 0.042; Tucker-Lewis index = 0.971; comparative fit index = 0.979; root mean square error of approximation = 0.061, 90% confidence interval = 0.035-0084). Higher COST scores were significantly correlated with higher health-related quality of life (EQ-5D-5L utility score: r = 0.21, p = 0.002; EQ VAS: r = 0.28, p < 0.001). Multivariate quantile regression analysis revealed that female sex, rural residence, and unstable job/unemployment were associated with lower COST scores. There was no statistically significant difference in other factors, including clinical factors (types of cancer, staging, and treatment modalities).

Conclusions: The COST is reliable and valid, making it suitable for assessing FT severity in Vietnamese patients with cancer.