What is the correct FAST ultrasound technique in veterinary patients and how do I interpret free fluid?

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Global Focused Assessment with Sonography for Trauma (Global FAST) combines Abdominal FAST (AFAST), Thoracic FAST (TFAST), and Vet BLUE into a single 15-data-point point-of-care ultrasound examination performed as an extension of the physical examination. This standardized, unbiased approach prevents the two most common interpretation errors in selective imaging: satisfaction of search error and confirmation bias error. 6 --- AFAST uses 5 acoustic windows across the abdomen and serves as a rapid screening test for free fluid, retroperitoneal fluid, pleural effusion, and pericardial effusion, as well as soft tissue abnormalities via a target-organ approach. 3 The four named quadrants assessed are the diaphragmatic-hepatic (DH), hepatorenal, splenorenal, and cystocolic views. 10 Add-on assessments at these same windows — without acquiring additional views — include caudal vena cava and hepatic vein evaluation for volume status, urinary bladder volume estimation for urine output tracking, screening for free air (pneumoperitoneum and pneumoretroperitoneum), and gastrointestinal motility assessment. 3 AFAST incorporates an abdominal fluid scoring system that semiquantitates free fluid volume across the acoustic windows, enabling serial monitoring of fluid accumulation or resolution over time. 3 When free fluid is detected at the DH view, abdominocentesis can be performed and the fluid characterized — for example, turbid yellow-to-brown fluid cytologically consistent with bile 5, or clear yellow fluid with total solids below the reference interval of 2.5 g/dL and peritoneal lactate above the reference interval of 2.0 mmol/L. 2 TFAST uses 5 acoustic windows: bilaterally applied chest tube site views, bilaterally applied pericardial site views, and the diaphragmatic-hepatic view shared with AFAST. It is validated for rapid detection of pneumothorax, pleural effusion, and pericardial effusion in dogs and cats. 11 A defined set of TFAST rules governs image interpretation to p