Algorithm for FAST

FAST is no substitute for CT and a more definitive assessment of the patient is obtained with CT.

Indeed, a positive FAST scan in a stable patient should always lead to a CT scan. This defines with precision where the pathology is within the peritoneal cavity, and furnishes the surgeon with as much information as possible. 

A positive scan in an unstable patient may result in the surgeon feeling that immediate laparotomy is appropriate. This depends on the experience of the surgeon and a degree of confidence that the fluid is not due to pelvis venous bleeding, ruptured hollow viscus, etc.