When is a Focused Assessment with Sonography for Trauma (FAsT) exam recommended?

When is a FAST Exam Recommended?

FAST should be performed immediately upon hospital arrival in all patients with suspected torso trauma, particularly those with hemodynamic instability or hemorrhagic shock with an unidentified bleeding source. 1

Primary Indications for FAST

Hemodynamically Unstable Patients

• Perform FAST immediately in patients presenting with hemorrhagic shock and an unidentified source of bleeding to rapidly detect free intraperitoneal or pericardial fluid. 1
• FAST is specifically recommended for hemodynamically unstable patients with blunt abdominal trauma to guide urgent surgical decision-making. 2
• Patients with significant free intra-abdominal fluid on FAST and hemodynamic instability should proceed directly to urgent surgery without delay. 1

Hemodynamically Stable Patients

• FAST should be performed on admission to the hospital in all patients with suspected abdominal trauma, even if hemodynamically stable, to rule in or out free intra-abdominal fluid. 1
• A positive FAST in stable patients provides valuable prognostic information and is strongly associated with adverse outcomes (positive likelihood ratio 34.3). 3
• FAST should not be omitted in stable patients with blunt abdominal trauma, as it provides risk stratification at minimal cost. 3

Specific Clinical Scenarios

• Extended FAST (E-FAST) is recommended in all patients with suspected severe trauma, including those with severe pelvic trauma, to detect hemopericardium, hemoperitoneum, hemothorax, and pneumothorax. 1
• FAST is indicated for penetrating trauma of the thoracoabdominal transition where there is uncertainty about abdominal cavity penetration. 2
• Perform FAST in any patient with hemodynamic instability of unknown cause, even outside the traditional trauma setting. 4

What FAST Can and Cannot Do

FAST Capabilities

• FAST reliably rules in free intra-abdominal fluid with high specificity (96-99%) and positive predictive value (97%). 1, 5
• FAST can rule out more than 500 ml of free fluid if the exam is negative. 1
• The exam has sensitivity of 67-76% and specificity of 84-99% for detecting intraperitoneal free fluid. 5, 6

Critical Limitations

• FAST does NOT rule out specific intra-abdominal organ injuries - it only detects free fluid. 1
• FAST does NOT identify retroperitoneal hemorrhage, which is particularly important in pelvic trauma. 1, 7
• A negative FAST does not exclude slowly accumulating intra-abdominal free fluid, especially if performed very early. 1
• FAST has low sensitivity (56-71%) for detecting specific organ injuries and may miss small volumes of free fluid (<500 ml). 1, 7
• FAST cannot characterize the nature of free fluid (blood vs. ascites vs. urine). 1

Clinical Decision Algorithm

For Unstable Patients:

1. Perform FAST immediately during resuscitation (average time 2-3 minutes). 1, 6
2. If FAST is positive with significant free fluid → proceed directly to operating room without CT scan. 1, 2
3. If FAST is negative but shock persists → perform urgent CT or proceed to surgery based on clinical suspicion. 1

For Stable Patients:

1. Perform FAST on admission to detect free fluid and risk stratify. 1, 3
2. If FAST is positive → proceed to contrast-enhanced CT to characterize injuries. 1
3. If FAST is negative → still perform CT if mechanism or clinical findings suggest significant trauma, as FAST cannot exclude organ injuries or retroperitoneal bleeding. 1, 7

Common Pitfalls to Avoid

• Do not delay surgical intervention in unstable patients with positive FAST to obtain additional imaging - this is a critical error that increases mortality. 1, 2
• Do not rely on negative FAST alone to exclude significant injury - CT is still needed in stable patients with concerning mechanisms or clinical findings. 1, 5
• Do not perform prehospital FAST to guide triage decisions - evidence does not support this, and it should never delay rapid transfer to a trauma center. 1
• Recognize that retroperitoneal hematomas (especially from pelvic fractures) can cause false-positive FAST by creating intraperitoneal fluid suffusion. 1
• Be aware that perinephric fat, bowel contents, and epicardial fat pads can be mistaken for free fluid. 1