Background
Functional capacity refers to an individual's ability to perform daily activities essential for independent living and social participation. The Functional Assessment Short Test (FAST) is widely used to assess functional impairment across multiple domains, particularly in bipolar disorder.

Objective
To psychometrically validate the FAST in the non-clinical population.

Methods
A total of 1,560 young adults were assessed through standardized interviews. Exploratory and confirmatory factor analyses were conducted using polychoric matrices and robust estimators.

Results
Exploratory analysis identified a five-factor structure (Autonomy, Work, Cognition, Finances, and Interpersonal Relations/Leisure), with excellent sampling adequacy (KMO = 0.919) and 63% explained variance. Confirmatory factor analysis demonstrated excellent model fit: Comparative Fit Index (CFI) = 0.957, Tucker-Lewis Index (TLI) = 0.952, Root Mean Square Error of Approximation (RMSEA) = 0.038 (90% confidence interval: 0.035–0.041), Standardized Root Mean Square Residual (SRMR) = 0.050, and Goodness of Fit Index (GFI) = 0.993. Factor loadings ranged from 0.432 to 0.887, and internal consistency was high (α = 0.871; ω = 0.892). Convergent validity was supported by a moderate negative correlation with the mental component of the SF-12 (ρ = -0.445; p < 0.001). 

Conclusions
The findings provide robust evidence of validity and reliability of the FAST in the non-clinical population, supporting further investigation of its use in mental health research and preventive settings involving young adults in Southern Brazil.