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Objectives and questions: The agreement between FAST and computed tomography for the detection of intraperitoneal free fluid in polytrauma remains incompletely understood, particularly regarding how temporal factors and fluid volume influence concordance. Although FAST is widely used during initial trauma assessment, its diagnostic performance compared with CT is reported to show considerable variability in the literature.

Material and methods: An observational study was conducted in a level I trauma centre, including polytrauma patients who underwent both FAST and CT on admission. Agreement was evaluated using overall concordance, positive and negative agreement, Cohen’s kappa with bootstrap confidence intervals, and Gwet’s AC1. The influence of the time from injury to admission and the CT free-fluid scale on discordance was examined using logistic regression. Statistical significance was set at P < 0.05.

Results: Among 228 patients, overall agreement between FAST and CT was 0.763, with a positive agreement of 0.829 and a negative agreement of 0.614. Cohen’s kappa was 0.45 (95 per cent confidence interval 0.33 to 0.57), and Gwet’s AC1 was 0.44 (95 per cent confidence interval 0.31 to 0.57). The chi-square test confirmed a significant association between modalities (χ² = 46.32, P = 1.0 × 10⁻¹¹). Logistic regression showed that neither time from injury to admission (odds ratio 1.00, P = 0.67) nor the semiquantitative CT free fluid scale (odds ratio 0.74, P = 0.12) significantly predicted discordance between FAST and CT.

Discussion and conclusions: FAST and CT demonstrate moderate concordance for identifying intraperitoneal free fluid in polytrauma patients. While FAST retains high confirmatory reliability when positive, discordance persists and is not significantly influenced by prehospital delay or fluid volume. FAST, therefore, remains an important adjunct in early trauma assessment, although CT continues to represent the definitive modality for abdominal evaluation.