This FAST exam demonstrates a large heterogenous fluid collection adjacent to the splenorenal space, but the devil is in the details. This collection is noted to have hyperechoic flecks throughout. Additionally, there is no evidence of fluid between the spleen and diaphram, nor between the spleen and left kidney. There is no evidence of hypoechoic free fluid in Morison’s Pouch or in the pelvis.
If a large fluid collection is seen adjacent to the spleen, one would expect to see fluid tracking to the more dependent area above the spleen if it were truly free peritoneal fluid. This potential FAST pitfall has been coined the “gastric fluid” sign.1
Although hemorrhagic free fluid can have a heterogenous appearance once the blood begins to clot, the speckled appearance of this fluid collection is typical for stomach or bowel contents.
The reported specificity of the FAST exam for free fluid is >98%. Other false positive findings that have been described: perinephric and pericardial fat pads, as well as hemothorax.1,2
It is important to be aware of this “gastric fluid” false-positive pitfall that could result in unnecessary laparotomies.
Nagdev A, Racht J. The “gastric fluid” sign: an unrecognized false-positive finding during focused assessment for trauma examinations. Am J Emerg Med. 2008;26(5):630.e5-7. [pubmed]