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Background: Emergency physicians commonly perform Focused Assessment with Sonography for Trauma (FAST) examinations to evaluate for free intraperitoneal fluid. Many ultrasound findings can be misinterpreted as free fluid, resulting in false-positive FAST examinations. Objectives: To describe a previously unreported ultrasound finding that can be misinterpreted as free intraperitoneal fluid. Case Report: A 32-year-old man was stabbed in the left upper abdomen. A FAST examination was performed and a right perinephric fat pad was interpreted as showing free fluid in Morison's pouch. After transfer to a trauma center, a repeat FAST examination revealed no signs of intraperitoneal free fluid. Wound exploration showed no signs of penetration into the peritoneal cavity. Conclusions: When performing a FAST examination, a wedge-shaped hypoechoic area in Morison's pouch that is bounded on both sides by echogenic lines (the “FAST Double-Line Sign”) is likely to represent perinephric fat and may result in a false-positive FAST examination.

This case report describes a patient who presented to the Emergency Department (ED) after a high-speed motor vehicle crash (MVC), whose initial ultrasound examination was interpreted as being positive for fluid in Morison’s pouch. Subsequent ultrasound examinations and computed tomography scans further delineated this finding to be fluid-filled bowel juxtaposed between the liver and right kidney. With greater implementation of ED ultrasound, it is important to identify entities that cause false-positive and false-negative examinations.