FAST & eFAST
- FAST (Focused Assessment with Sonography in Trauma) is a fast bedside ultrasound that hunts for free fluid — in trauma, that usually means blood — in the spaces where it likes to pool.
- The classic four windows: right upper quadrant, left upper quadrant, the pelvis, and a peek at the heart.
- "eFAST" tacks on a look at the lungs to catch a pneumothorax and hemothorax — the "e" is for extended.
- A positive FAST in an unstable trauma patient is a fast track to the operating room. A negative FAST never fully rules out injury.
- Free fluid shows up as black (anechoic) stripes collecting in the gravity-dependent crevices between organs.
Imagine a trauma patient rolls in, blood pressure tanking, and you have about ninety seconds to figure out whether they're bleeding into their belly. You can't exactly pause to schedule a CT. This is the moment the FAST exam was built for: a quick, no-radiation, do-it-at-the-bedside ultrasound that answers one blunt question — is there free fluid where there shouldn't be?
That's really all FAST is. It's not a fishing trip for every possible injury. It's a smoke detector. It doesn't tell you the house is fine; it tells you whether there's smoke.
What the letters actually mean
FAST stands for Focused Assessment with Sonography in Trauma. Every word earns its place: focused (we're answering one question, not doing a full anatomy tour), sonography (ultrasound), in trauma (the use case it was designed for). If ultrasound knobology is the alphabet, FAST is a single very useful word.
The eFAST — extended FAST — adds a look at the lungs. Same exam, two extra stops, and suddenly you can also catch a collapsed lung and blood in the chest. We'll get there.
Where the fluid hides
Here's the trick that makes FAST work: blood obeys gravity. In a patient lying on their back, fluid drains into the lowest, most dependent pockets of the abdomen and settles there like rainwater pooling in the dips of a parking lot. FAST just parks the probe over those dips.
The exam has four standard windows:
| Window | What you're looking at | Where fluid collects |
|---|---|---|
| Right upper quadrant (RUQ) | Liver & right kidney | The space between them — Morison's pouch |
| Left upper quadrant (LUQ) | Spleen & left kidney | The splenorenal recess and above the spleen |
| Pelvis | Bladder | Behind/around the bladder, the most dependent spot |
| Cardiac (subxiphoid) | Heart in its sac | The pericardium |
The RUQ view, looking at the potential space between the liver and right kidney (radiologists call it the hepatorenal recess, or Morison's pouch), is the single highest-yield spot — supine blood loves to pool there first. If I only got one window, that's the one I'd take.
A classic beginner trap in the LUQ: blood often collects above the spleen (between spleen and diaphragm) before it shows up in the splenorenal recess. Hunt high, or you'll walk right past it.
What free fluid looks like
On ultrasound, fluid is lazy — it doesn't bounce sound back, so it shows up black (the term is anechoic). You're looking for a dark stripe or wedge slipping into a space that's normally pressed shut, sharpening the crease between two organs into a thin black line.
The catch is that black is not automatically blood. A full bladder, fluid-filled bowel, a cyst, or ascites are all black too. FAST tells you fluid is present; it does not read the fluid's mind about where it came from. Context — a trauma patient, a falling blood pressure — is what turns "free fluid" into "probably hemorrhage."
Don't confuse FAST with a complete trauma workup. A negative FAST does not rule out solid-organ injury, bowel injury, or retroperitoneal bleeding — those can be present with little or no free fluid early on. A negative FAST in a stable patient still often earns a CT for the full picture.
The "e" in eFAST: looking at the chest
The extended exam adds two lung views, and they punch above their weight.
First, pneumothorax — air leaking into the space around the lung. On a normal lung, the two pleural layers slide against each other with each breath, a shimmer called lung sliding. Air in the way kills the shimmer. No sliding = suspect a pneumothorax. Ultrasound is genuinely good at this, often beating a supine chest X-ray. (The scariest version, tension pneumothorax, is a clinical emergency you treat first and image never.)
Second, hemothorax — blood in the chest cavity, which once again shows up as that telltale black collection above the diaphragm, the thoracic cousin of the pleural effusion you'd see on a chest film.
The cardiac window and a word on what FAST isn't
The subxiphoid heart view is checking for fluid in the pericardium — the sac around the heart. A growing black rim there can signal blood compressing the heart, the situation behind pericardial disease called tamponade.
FAST is fastest and most decisive in the unstable patient: a positive scan plus a crashing blood pressure can send someone to the operating room without ever passing through the CT scanner. In the stable patient, it's a triage tool that helps decide who needs the scanner next.
So here's the one thing to walk away with: FAST is a yes/no smoke detector for free fluid, read in the context of the patient in front of you. It's quick, it's repeatable, and it spares people radiation — but a quiet alarm doesn't mean the building is safe. When in doubt, scan again, or send them for the full picture.