FAST Scan Detection Threshold for Free Intraperitoneal Fluid

Minimum Detectable Volume

A negative FAST examination cannot reliably rule out volumes less than 500 mL of free intraperitoneal fluid in the abdominal cavity, as stated by the European Society of Trauma and Emergency Surgery guidelines 1
The 500 mL threshold represents the clinical reliability cutoff—smaller volumes may be missed, particularly in early trauma presentations before fluid has adequately accumulated 1, 2

FAST cannot identify specific organ injuries or characterize the nature of free fluid, such as blood versus ascites versus urine 1
FAST 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, 2
Sensitivity ranges from 74-88% with specificity of 96-99%, meaning approximately 1 in 4 injuries may be missed 1, 3, 4

CT imaging can reliably detect 100-250 mL of free fluid with 98% sensitivity and specificity for solid organ injuries, although this is not from the primary guideline society, it is mentioned for comparison 2
Diagnostic peritoneal lavage (DPL) remains most sensitive, detecting as little as 20 mL of intraperitoneal blood, as per the comparison with other modalities 2
FAST offers the advantage of being rapid, non-invasive, repeatable, and performable at the bedside during resuscitation, as noted in the Annals of emergency medicine 3, 4

For hemodynamically unstable patients, a positive FAST leads to immediate laparotomy, while a negative FAST does not exclude injury and requires alternative diagnostic strategies or repeat examination, as per the Critical Care guidelines 1, 2, 5, 6
For hemodynamically stable patients, a positive FAST leads to CT for detailed injury characterization, and a negative FAST still indicates CT if clinical suspicion persists, due to the potential for missing volumes less than 500 mL 1, 2, 5, 6