• Unstable blunt trauma: a positive abdominal FAST in a patient who does not respond to resuscitation is an indication for laparotomy (or damage-control surgery) without CT. In hypotensive blunt trauma patients scanned by experienced surgeons, sensitivity and specificity were both 100% (Rozycki 1998): a best case from an expert centre, not a guarantee. Unstable here means a transient or non-responder to the initial fluid or blood bolus (ATLS).
• Stable blunt trauma: a positive FAST speeds CT and surgical review; a negative FAST does not end the work-up. CT remains the reference for the stable patient.
• Penetrating thoracoabdominal trauma: the pericardial view is the key window. Pericardial blood after a precordial or epigastric stab wound means surgical exploration (pericardial view sensitivity 100%, specificity 96.9% in 261 patients, Rozycki 1999; but see Ball 2009 on false negatives with a haemothorax). Abdominal FAST has low sensitivity here: a positive result is useful; a negative result rules out nothing.
• Chest: ultrasound of the anterior chest is more sensitive for pneumothorax than a supine chest X-ray (Alrajhi 2012: sensitivity 90.9% vs 50.2%, specificity 98.2% vs 99.4%).
• Serial scanning: repeat after any change in haemodynamics. An early negative scan may become positive as blood accumulates.

• Retroperitoneal haemorrhage: pelvic fracture bleeding, renal injury, major vessel injury. Blood stays behind the peritoneum and never reaches Morison's pouch.
• Hollow viscus and mesenteric injury: bowel perforation produces little free fluid early.
• Solid organ injury without free fluid: a contained subcapsular haematoma or parenchymal laceration. The FAST looks for fluid, not organs.
• Diaphragmatic injury.
• Small volumes of haemoperitoneum below the detection threshold, especially early.
• Clotted blood: clot is echogenic and can look like tissue.

• Pelvis (rectovesical pouch in males; pouch of Douglas in females): the most dependent part of the pelvic peritoneal cavity; with Morison's pouch, one of the two main sumps in the supine patient. It detects the smallest volumes in DPL-infusion studies (Von Kuenssberg Jehle 2003: median about 100 mL), but in real trauma the RUQ, especially the caudal liver tip, is the view most often positive (Lobo 2017).
• Hepatorenal recess (Morison's pouch): the most dependent upper abdominal space; the right paracolic gutter connects it to the pelvis.
• Left upper quadrant: in positive FASTs with LUQ fluid, 84% had fluid in the left paracolic gutter, and fluid confined to the LUQ (6 of 100 positive scans) lay below the spleen tip and in the gutter, or in the subphrenic space above the spleen, never in the splenorenal recess alone (O'Brien 2015). The phrenicocolic ligament partly separates the left subphrenic space from the paracolic gutter, so blood can sit above the spleen without reaching the gutter, or the other way round; in O'Brien's series the gutter was the zone most often positive. Clear all of these zones, not just between spleen and kidney.

The caudal tip of the liver and the adjacent right paracolic gutter are the RUQ sub-zone most often positive, more than Morison's pouch proper (30 of 32 positive RUQ views; Lobo 2017). Always slide caudally to see the liver tip.

Detection thresholds vary by study and operator. For a supine Morison's pouch view the mean was about 600 mL in Branney 1995, though some patients were detected at much smaller volumes; the pelvic view detects smaller volumes. A negative FAST can coexist with several hundred mL of blood.

Trendelenburg (head-down) moves fluid towards the upper abdomen and improves detection in Morison's pouch (Abrams 1999); reverse Trendelenburg favours the pelvis. Reposition only if the patient and spinal precautions allow.

• Probe more posterior and superior than on the right: posterior axillary line, about 6th–9th intercostal spaces, knuckles on the bed. The spleen is small and sits high and back; stomach and colon gas sit anteriorly.
• Clear: left hemithorax (mirror image, spine sign), subphrenic space (missed by a probe too low and anterior), splenorenal recess, inferior tip of the spleen and left paracolic gutter (where isolated LUQ fluid was most often found; O'Brien 2015).

• Just above the pubic symphysis, angled caudally. Transverse and sagittal, fanning fully through the bladder.
• Males: fluid in the rectovesical pouch, behind the bladder. Females: in the pouch of Douglas, behind the uterus (and it can lie between bladder and uterus).
• A full bladder is the acoustic window: scan before urinary catheterisation, or clamp the catheter. Fill with saline only via an existing, correctly placed catheter, and not if bladder or urethral injury is suspected (pelvic fracture, haematuria, blood at the meatus): instilled fluid can leak through a ruptured bladder and mimic haemoperitoneum. Otherwise record the pelvis as indeterminate and move on; do not delay resuscitation.
• A small amount of free pelvic fluid can be physiological in women of reproductive age; interpret in context.

• Subxiphoid: probe flat under the xiphisternum, aimed at the left shoulder, liver as the window; increase depth to see the posterior pericardium.
• If subxiphoid is blocked (tenderness, gas, obesity, a wound) use the parasternal long axis (focused cardiac). There, pericardial fluid lies anterior to the descending aorta; pleural fluid lies posterior to it.
• An isolated anterior echo-free space without posterior fluid is often epicardial fat.

• Lung sliding: shimmer of the pleural line with respiration.
• B-lines: they arise from the pleural line, so the visceral pleura (lung) is touching the chest wall.
• Lung pulse: the pleural line pulses with the heartbeat in an apnoeic or poorly ventilated lung.

Absent sliding, absent B-lines and absent lung pulse, with A-lines remaining. M-mode: seashore (granular below the pleural line) becomes barcode (horizontal lines throughout).

Lung point: where sliding lung meets the non-sliding pneumothorax. Specificity close to 100%, but absent in a total collapse.

• Perinephric fat: hypoechoic rim around the kidney, usually bilateral and symmetrical, does not track into the recess, has internal echoes.
• Seminal vesicles: paired, symmetrical bow-tie structures behind the male bladder; confirm in sagittal. Fluid is angular and conforms to spaces.
• Fluid-filled bowel: has a wall, contains debris and shows peristalsis.
• Gallbladder, IVC, stomach, renal pelvis, ovarian cyst: contained structures with walls; fan to see they are round or tubular.
• Ascites, dialysate, urine (bladder rupture): genuine fluid, but not blood. History matters.
• Epicardial fat pad: anterior only, mildly echogenic, moves with the heart.
• Stomach or pleural fluid confused with pericardial fluid.

• Too early, too little: scan again.
• Clotted blood is echogenic and can look like liver, spleen or bowel. Look for heterogeneous material in a dependent space.
• Incomplete views: liver tip not seen, LUQ too low or anterior, empty bladder, no fanning.
• Obesity, subcutaneous emphysema, bowel gas. Retroperitoneal bleeding (not visible by design).
• Pericardial: a haemothorax may decompress a pericardial laceration into the pleura. A negative pericardial view with a large left haemothorax in penetrating chest trauma does not exclude cardiac injury.

Haemorrhage control: laparotomy or damage-control surgery; activate massive transfusion; do not go to CT.

The bleeding is elsewhere or not yet visible: chest (haemothorax, tension pneumothorax); pericardium (tamponade); pelvis and retroperitoneum (binder, pelvic X-ray, angioembolisation or preperitoneal packing); long bones and external haemorrhage; non-haemorrhagic shock (tension, tamponade, spinal cord injury, cardiac contusion: RUSH). Repeat the FAST; consider diagnostic peritoneal aspirate or lavage if doubt remains and local practice supports it.

FAST positive: CT with contrast (non-operative management of solid organ injury may be possible); early surgical review. FAST negative: CT according to mechanism and examination; serial examination and repeat FAST. A negative FAST does not clear the abdomen.

Assume haemopericardium: urgent surgical decompression. Pericardiocentesis is only a bridge where surgery is not immediately available.

Tension features: immediate decompression. Otherwise drain according to size, ventilation (positive pressure, transfer, theatre) and local guidance.