Four abdominal windows and two thoracic ones, each answering one question — free fluid, pericardial fluid, pneumothorax. A positive FAST in an unstable patient changes the next step; a negative FAST excludes nothing.
Orient first
- FAST answers "is there free fluid?" — not "which organ is injured". Solid organ injuries without haemoperitoneum are invisible to it.
- Free fluid collects in the most dependent spaces: the hepatorenal pouch (Morison), the splenorenal recess and the space above the spleen, and the pelvis (rectovesical or rectouterine pouch).
- The extended FAST adds the anterior chest for pneumothorax: normal lung slides at the pleural line; a pneumothorax does not, and the lung point is where sliding resumes.
Acquire the study
- Curvilinear (low-frequency) probe for the abdominal and pericardial windows; linear or the same curvilinear probe at the anterior chest for lung sliding.
- Supine patient; a full bladder helps the pelvic view (before catheterisation where possible).
- Order: right upper quadrant, left upper quadrant, pelvis, subxiphoid (or parasternal) pericardium, then both anterior chests.
The manoeuvre
- Right upper quadrant: the hepatorenal interface, the inferior liver tip and the right paracolic gutter, and the right subdiaphragmatic space — anechoic fluid between liver and kidney is positive.
- Left upper quadrant: scan posteriorly and high; fluid collects above the spleen (subphrenic) before the splenorenal recess.
- Pelvis in transverse and sagittal: fluid behind the bladder (rectovesical) or the uterus (pouch of Douglas).
- Subxiphoid pericardial view, angled towards the left shoulder: an anechoic stripe around the heart; look for right ventricular diastolic collapse.
- Anterior chest, 2nd–4th intercostal space, probe perpendicular to the ribs: lung sliding at the pleural line, B-lines, and in M-mode the seashore versus barcode pattern; search laterally for the lung point.
- Repeat the examination when the patient's condition changes — a single negative FAST is a snapshot.
What confirms it
- Anechoic or low-level echogenic fluid in a dependent space on two planes; pericardial fluid on the subxiphoid view; absent sliding with a lung point for pneumothorax.
What licenses you to exclude it
- A negative FAST does not exclude solid organ, bowel, retroperitoneal or pelvic injury — the stable patient with a significant mechanism needs CT.
- Absent lung sliding alone is not specific (intubation of the other bronchus, adhesions); the lung point is.
The classic misread
- Calling perinephric fat, the gallbladder or seminal vesicles free fluid.
- Missing a small haemoperitoneum above the spleen by scanning only the splenorenal recess.
- Missing clotted blood, which is echogenic and can look like solid tissue.