What is the FAST exam and its limitations in the trauma patient?

• A. Focused assessment with sonography for trauma; detects pericardial and intraperitoneal free fluid; limited for retroperitoneal injuries and early solid organ injuries
• B. It replaces the need for CT scans in all trauma patients
• C. It detects brain bleeding
• D. It cannot detect free fluid

Answer: (A)

The FAST exam is a rapid, bedside ultrasound used in trauma to identify free fluid from bleeding. It focuses on the pericardial sac to assess for tamponade and on the upper abdominal spaces (such as the hepatorenal and splenorenal recesses) to detect intraperitoneal free fluid, helping guide immediate management, especially in an unstable patient. This test is a quick triage tool, not a complete survey of all injuries. It has clear limitations: it does not reliably assess retroperitoneal injuries (such as those involving the kidneys, pancreas, or duodenum) and may miss early or small-volume bleeding from solid organs before fluid accumulates. The exam is also operator dependent and can be limited by body habitus or bowel gas. Because it cannot replace detailed imaging in stable patients, CT remains the more definitive study when the patient can undergo it. In practice, a positive FAST supports urgent intervention, while a negative FAST does not completely rule out injury, especially retroperitoneal or early solid-organ injuries.