What is a FAST Exam?
The Focused Assessment with Sonography for Trauma (FAST) exam is a rapid, bedside ultrasound protocol performed in trauma patients to detect free fluid (blood) in the pericardium and peritoneum, serving primarily as a triage tool to identify life-threatening hemorrhage and guide immediate surgical decisions. 1
Primary Purpose and Clinical Role
FAST functions as a triage tool rather than a comprehensive diagnostic test. 1 The exam aims to answer a simple "yes or no" question: Is there free fluid present that likely represents bleeding? 2 It does not quantify bleeding volume or identify specific organ injuries. 3
The exam is performed during the initial resuscitation phase and typically takes 2-3 minutes to complete. 3 A positive FAST combined with hemodynamic instability (systolic blood pressure <90 mm Hg) should lead directly to the operating room without delay for additional imaging like CT. 1, 3
What FAST Examines
The traditional FAST protocol evaluates four specific anatomic windows: 1, 2
- Pericardial window (subcostal view) - detects hemopericardium and cardiac tamponade
- Right upper quadrant - evaluates Morrison's pouch (hepatorenal space) for free fluid
- Left upper quadrant - assesses the splenorenal space for free fluid
- Pelvis - examines the rectovesical or rectouterine pouch for free fluid
The extended FAST (E-FAST) protocol adds bilateral pleural space evaluation to detect hemothorax and pneumothorax. 1, 2
Diagnostic Performance
FAST demonstrates high specificity (96-99%) and positive predictive value (97%) for detecting free intra-abdominal fluid. 3 When positive, it reliably rules in the presence of significant bleeding. 3
However, FAST has important limitations in sensitivity. 4 It can detect free fluid volumes greater than 500 ml, but smaller amounts may be missed. 3 The sensitivity ranges from 56-80% depending on the clinical context and operator experience. 5, 4
Critical Limitations to Understand
FAST does NOT rule out specific organ injuries - it only detects free fluid. 3 A negative FAST does not exclude:
- Retroperitoneal hemorrhage (particularly important in pelvic trauma) 5
- Solid organ injuries without significant free fluid 1
- Bowel or mesenteric injuries 6
- Slowly accumulating bleeding if performed very early 1
A negative FAST in a hemodynamically stable patient still requires CT imaging if there is concerning mechanism of injury or clinical findings. 3 The exam has insufficient sensitivity to exclude significant injuries when negative. 1
Clinical Decision Algorithm
For hemodynamically unstable patients (systolic BP <90 mm Hg despite resuscitation): 1
- Perform FAST immediately during resuscitation
- If FAST is positive → proceed directly to operating room 3
- If FAST is negative → consider other sources of instability and repeat exam, as early negative results may miss slowly accumulating fluid 1
For hemodynamically stable patients: 1, 3
- Perform FAST on admission for risk stratification
- If FAST is positive → proceed to contrast-enhanced CT to characterize specific injuries 3
- If FAST is negative → still obtain CT if mechanism or clinical findings are concerning 3
Common Pitfalls to Avoid
Never delay surgical intervention in unstable patients with positive FAST to obtain additional imaging - this increases mortality. 3 The combination of positive FAST and hemodynamic instability is sufficient indication for immediate laparotomy.
Do not rely on FAST alone in normotensive patients. 1 The cardiac component of FAST may not be an effective use of resources for normotensive blunt trauma patients, as it has limited ability to detect certain injuries like right ventricular injuries or valvular pathology. 1
Recognize that FAST has lower diagnostic performance in the prehospital setting compared to in-hospital use, with feasibility impeded by poor conditions, obesity, and operator inexperience. 1
Remember that retroperitoneal hematomas (such as from pelvic fractures) can generate intra-abdominal free fluid, potentially leading to false localization of the bleeding source. 1