Volume-dependent detection of free intraperitoneal fluid by FAST: a prospective in-vivo study with sequential fluid instillation

PURPOSE: To assess the volume-dependent sensitivity of the Focused Assessment with Sonography for Trauma (FAST) and to analyse regional detection performance under controlled in-vivo conditions. METHODS: In this prospective single-centre study, sequential intraperitoneal instillation of 0.9% NaCl (250-1500 mL) was performed during elective laparoscopic procedures in 35 adults. Standardised FAST examinations were conducted by two experienced sonographers using transverse and longitudinal views of Morison's pouch, the Douglas recess, and Koller's space. Sensitivity was calculated for each instilled volume and anatomical region. Specificity could not be assessed due to the experimental design with mandatory fluid presence. Regional differences were tested using one-way ANOVA, and a Monte Carlo simulation (1000 resamples) evaluated detection stability across body-mass-index (BMI) groups. RESULTS: FAST sensitivity increased with volume: 14.3% (95% CI 4.8-30.3) at 250 mL, 37.1% (21.5-55.1) at 500 mL, 74.3% (56.7-87.5) at 1000 mL, and 91.4% (76.9-98.2) at 1500 mL. The highest detection rate occurred in Morison's pouch, followed by the Douglas and Koller recesses. Longitudinal scanning of the Douglas pouch improved sensitivity by > 20% compared with the transverse view. ANOVA confirmed significant regional variability (F(5, 153) = 4.37, p = 0.001, η² = 0.125). Detection rates declined in participants with BMI ≥ 30 kg/m² non-significantly, while instilled volume remained the dominant predictor (OR > 20 for 1500 mL vs. 250 mL). CONCLUSIONS: FAST sensitivity is strongly volume-dependent, with reliable detection achieved only beyond 1000 mL. Incorporating longitudinal pelvic views-particularly of the Douglas pouch-significantly enhances diagnostic accuracy. These findings support refinement of FAST training and underscore the need for clinical correlation and adjunct imaging when FAST results are negative in unstable trauma patients.