What amount of free fluid in the abdominal cavity can an ultrasound fast scan detect?

FAST Scan Detection Threshold for Free Intraperitoneal Fluid

A negative FAST examination cannot reliably rule out volumes less than 500 mL of free intraperitoneal fluid in the abdominal cavity. 1

Minimum Detectable Volume

• FAST can detect approximately 100-250 mL of free fluid in the pelvic views, with a median detection threshold of 100 mL in prospective studies. 2
• The European Society of Trauma and Emergency Surgery guidelines explicitly state that a negative FAST does not rule out amounts less than 500 mL of free fluid. 1
• This 500 mL threshold represents the clinical reliability cutoff—smaller volumes may be missed, particularly in early trauma presentations before fluid has adequately accumulated. 1, 3

Anatomic Considerations for Fluid Detection

• The caudal edge of the liver and superior paracolic gutter (RUQ3 sub-quadrant) is the most sensitive location for detecting free fluid, with 93.8% of positive RUQ findings identified in this area. 4
• The right upper quadrant overall is the most sensitive view, detecting fluid in 66.7% of positive FAST examinations. 4
• The pelvic views demonstrate superior sensitivity compared to single-view RUQ examinations, detecting a mean of 129-157 mL of fluid. 2

Critical Clinical Limitations

FAST has important diagnostic blind spots that clinicians must recognize:

• Cannot identify specific organ injuries or characterize the nature of free fluid (blood versus ascites versus urine). 1
• Cannot detect retroperitoneal hematomas, which is particularly problematic in patients with pelvic fractures. 1
• Early negative results do not exclude slowly accumulating intraperitoneal fluid, as fluid takes time to collect in detectable quantities. 1, 3
• Sensitivity ranges from 74-88% with specificity of 96-99%, meaning approximately 1 in 4 injuries may be missed. 1

Comparison with Other Modalities

• CT imaging can reliably detect 100-250 mL of free fluid with 98% sensitivity and specificity for solid organ injuries. 3
• Diagnostic peritoneal lavage (DPL) remains most sensitive, detecting as little as 20 mL of intraperitoneal blood. 3
• However, FAST offers the advantage of being rapid, non-invasive, repeatable, and performable at the bedside during resuscitation. 1

Clinical Application Algorithm

For hemodynamically unstable patients (SBP <90 mmHg):

• Positive FAST → immediate laparotomy (sensitivity and specificity approach 100% in this population). 1, 5
• Negative FAST → does not exclude injury; proceed to alternative diagnostic strategy or repeat examination. 1, 3

For hemodynamically stable patients:

• Positive FAST → proceed to CT for detailed injury characterization and potential non-operative management. 1, 5
• Negative FAST → CT remains indicated if clinical suspicion persists, as volumes <500 mL may be missed. 1, 3