Prevalence of Asymptomatic Intraperitoneal Fluid in Children on Focused Assessment with Sonography in Trauma (FAST) Ultrasound

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Background and Objective

Focused Assessment with Sonography in Trauma (FAST) exam is a commonly used ultrasound in pediatric patients to assess for intra-abdominal injuries in cases of blunt trauma. The detection of free fluid in the abdomen in these clinical scenarios is presumed to be intraperitoneal bleeding, however small amounts of fluid can be present physiologically, which introduces uncertainty in interpreting FAST exams. Classic teaching is that this occurs mostly in menstruating females, though it has also been reported in males and younger children. Information regarding prevalence and amount of physiologic free fluid in different age groups and genders is not well studied. Therefore, we sought to study the prevalence and amount of free fluid on FAST exams in asymptomatic children.


A convenience sample of children ages 2 to 18 years presenting to the pediatric emergency department for non-abdominopelvic complaints from August to December 2019 was enrolled. Potential subjects were excluded if they had recent abdominal trauma, surgery, or a known medical condition that could predispose to intraperitoneal free fluid. Three FAST exam views (right upper quadrant, left upper quadrant, and pelvis) were obtained using a Sonosite X-Porte (FUJIFILM Sonosite, Inc., Bothell, Washington) with a curvilinear probe to determine presence of free fluid. The amount of free fluid was calculated using the volume function on the ultrasound machine. Presence of free fluid was determined as a prevalence with 95% confidence interval (CI).


We enrolled 52 children, and 50 were included. Two were excluded for incomplete ultrasound examinations. The average age of children enrolled was 10.1 years, the average weight was 21.6 kilograms, and 31 (62%) were male. Free fluid was identified in the pelvis in 8 children (16%; 95%CI 8.3%-28.5%), 5 of whom were male, with an average age of 8.1 years old (95% CI 5.5-10.7 years). The average volume of fluid was 0.6mL with the maximum of 1.6mL (example shown in Figure 1).  No free fluid was identified in the upper quadrants in any subject.


Small amounts of pelvic free fluid is common in asymptomatic children, including in males. Prospective study is warranted in trauma patients to determine if such small amounts of pelvic free fluid are significant in pediatric abdominal trauma.

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Penn State Hershey Medical Center
University of Maryland Medical Center
Penn State Health Hershey Medical Center

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