The FAST Exam
- FAST = Focused Assessment with Sonography for Trauma. It's a quick bedside ultrasound that asks one blunt question: is there free fluid (read: blood) where there shouldn't be?
- It checks four spots — around the liver, around the spleen, behind the bladder, and around the heart — looking for black stripes of fluid in places that are normally tidy.
- A positive FAST in an unstable trauma patient sends them to the operating room, fast. A negative FAST does not mean "all clear."
- The eFAST adds a look at the lungs for pneumothorax and hemothorax.
- It finds fluid, not the bleeding source — it's a triage tool, not the final answer.
Imagine you've spilled a glass of water on a cluttered desk. You don't see the water itself first — you see it pooling in the seams, the low corners, the gap behind the monitor. The FAST exam is exactly that, but the "desk" is a trauma patient's belly and the "water" is blood. Ultrasound can't tell you where the leak is, but it's brilliant at spotting the puddle collecting in the low corners. And in a crashing trauma patient, knowing there's a puddle is often all you need to act.
What it actually is
FAST stands for Focused Assessment with Sonography for Trauma. The key word is focused — this isn't a leisurely tour of the abdomen. It's a handful of targeted looks, done in a couple of minutes, usually right in the resuscitation bay while everyone else is sticking lines and asking what happened.
It's a flavor of point-of-care ultrasound, and like all ultrasound it lives and dies by the basics of probe handling and knobology. The physics that make fluid show up black are the same ones covered in ultrasound physics: fluid doesn't bounce sound back, so it returns no echoes and paints as anechoic — pure black — on the screen.
On ultrasound, free fluid is black (anechoic). You're hunting for black stripes wedged into spaces that should be snug.
The four windows
The classic FAST looks at four areas. The logic is simple: blood obeys gravity, so you go where it pools.
| Window | Where you point the probe | What you're looking for |
|---|---|---|
| Right upper quadrant (Morison's pouch) | Between the liver and right kidney | A black stripe in the potential space between them |
| Left upper quadrant | Between the spleen and left kidney (and above the spleen) | Fluid around the spleen — it often collects above it first |
| Pelvis | Behind the bladder | Fluid tracking down into the deepest part of the pelvis |
| Subxiphoid (cardiac) | Just under the breastbone, aimed up at the heart | Fluid in the pericardial sac around the heart |
That right-upper-quadrant view deserves a name-drop: the space between liver and kidney is Morison's pouch (the hepatorenal recess), and it's one of the most dependent spots in a supine person's abdomen. Lay someone flat and blood from all over the upper abdomen tends to trickle there. It's the single highest-yield view, which is why it usually goes first.
The cardiac window
The subxiphoid view is the odd one out — it's not about the abdomen at all. You're looking at the pericardium, the sac wrapped around the heart. Blood in that sac (a pericardial effusion, or in trauma, hemopericardium) can squeeze the heart so it can't fill — cardiac tamponade. That's a different emergency with a different fix, and it's exactly the kind of thing you want to know about in the first two minutes.
A swirl of black around the heart on the subxiphoid view, in a hypotensive trauma patient, is tamponade until proven otherwise. This is a reach-for-the-phone finding, not a "let's image more" finding.
The "e" in eFAST
Tack an "e" on the front — eFAST, the extended FAST — and you add a look at the lungs. Sliding the probe up onto the chest wall lets you check for two things trauma loves to cause: a pneumothorax (air in the wrong place, which abolishes the normal shimmer of lung sliding) and a hemothorax, which is just a pleural effusion made of blood — more black fluid, now sitting above the diaphragm.
How to read it — and where it lies
Here's the honest part, because accuracy beats a clean story. FAST is great at answering one question and terrible at most others.
A positive FAST — free fluid — in a patient whose blood pressure is in the basement is a powerful signal. In an unstable trauma patient, that often means surgery now, no CT detour. The ultrasound paid for itself in ninety seconds.
A negative FAST is where people get burned. It does not rule out injury. Ultrasound is poor at seeing injuries to the organ tissue itself, and it needs a fair amount of fluid before it lights up — a small or early bleed can look perfectly black-free. It's also blind to most injuries behind the abdominal cavity (the retroperitoneum) and to hollow-bowel injury.
A negative FAST never "clears" a patient. If they're stable, the real workup is usually a trauma (polytrauma) CT; if they're unstable but FAST is negative, the bleeding may be somewhere FAST can't see. Don't let a black-free screen lull you.
A few more honest gotchas worth knowing: a little fluid in the pelvis can be physiologic in some patients and isn't always blood; fluid that's already in the body — ascites, urine from a full bladder, even prior dialysis fluid — can fool you into a false positive; and a patient with too much bowel gas or a large body habitus can be genuinely hard to image. Ultrasound rewards the honest operator who knows what the picture can and can't say.
The one thing to remember
FAST is a triage tool, not a diagnosis. It answers "is there free fluid?" with speed and decent reliability when the answer is yes — and that single yes/no, in the right unstable patient, can change the next five minutes of their care. Treat a positive as real and act on it; treat a negative as "keep looking," never as "all clear."