In a penetrating injury, should I perform a FAST (Focused Assessment with Sonography for Trauma) exam?

Yes, FAST is Valuable in Penetrating Injury

FAST should be performed in penetrating torso trauma as it is a highly specific (94-100%) bedside tool that helps triage patients and directly guides surgical decision-making, particularly in hypotensive patients where detecting hemopericardium, pneumothorax, or free intraperitoneal fluid has immediate management implications 1.

Clinical Algorithm Based on Hemodynamic Status

Hypotensive Patients (SBP <90 mmHg or HR outside 50-110 bpm)

• Perform FAST immediately at bedside alongside portable chest/pelvic radiographs 1
• FAST findings directly determine next steps:
  • Positive FAST (hemopericardium, free fluid, or pneumothorax) → Proceed directly to operating room 1
  • Negative FAST → Consider CT if patient stabilizes during resuscitation, but degree of instability and distance to CT scanner must be factored 1
• Some trauma systems use prehospital FAST to expedite management upon hospital arrival 1

Hemodynamically Stable Patients

• FAST remains useful but typically performed after initial triage, followed by CT chest/abdomen/pelvis with IV contrast for definitive evaluation 1
• One study showed in-hospital mortality was not significantly different when FAST was bypassed in stable ballistic trauma patients, though this doesn't negate its utility 1

Understanding FAST Performance in Penetrating Trauma

Key Strengths

• Specificity is excellent (94.1-100%) across all studies 2
• Positive predictive value is high (92%) - a positive FAST reliably indicates injury requiring surgery 3
• For cardiac injury specifically: 100% sensitivity and specificity for pericardial effusion, with 87.5% requiring thoracotomy 4
• For intraperitoneal injury: 100% sensitivity and specificity when fluid is present without pericardial effusion 4

Critical Limitations

• Sensitivity is poor (28.1-100%, typically <50% for thoracic views) 2, 5
• Particularly low for:
  • Pneumothorax: 22% sensitivity 5
  • Hemothorax: 36% sensitivity 5
  • Peritoneal fluid: 51% sensitivity 5
• A negative FAST does NOT exclude injury - diaphragm and hollow viscus injuries are commonly missed 3

The Clinical Bottom Line

A positive FAST should prompt immediate laparotomy or thoracotomy 2, 3, 4. The high specificity means false positives are rare, making it an excellent "rule-in" test.

A negative FAST requires further workup - proceed with local wound exploration, CT imaging, diagnostic peritoneal lavage, or serial examinations depending on clinical context 2, 3. Over 70% of patients with false-negative FAST for pneumothorax or hemothorax still required chest tube placement 5.

Common Pitfalls to Avoid

1. Never rely on negative FAST alone to exclude injury in penetrating trauma - the sensitivity is too low 2, 5, 3
2. Don't delay surgery in unstable patients to obtain FAST if clinical signs already mandate operative intervention 1
3. Recognize mechanism matters: Ballistic injuries carry higher energy and risk of occult injuries that FAST may miss compared to stab wounds 1
4. Serial FAST examinations may improve detection in equivocal cases, though this is not well-studied in penetrating trauma 3

The 2024 ACR Appropriateness Criteria clearly position FAST as a complementary bedside tool that, when positive, has "significant implications on next steps in management" but should be integrated with other imaging modalities rather than used in isolation 1.