Free fluid

Free fluid looks black, both in the peritoneal cavity and pleural recess. Early peritoneal collections are seen just anterior to the renal capsule and appear as a black stripe.

Free fluid example

A scan showing free fluid in the peritoneal cavity and in the pleural recess.

Small pleural collections

Small pleural collections begin to accumulate posteriorly, therefore may not be seen during a FAST scan. In the image shown, the FAST appearances were normal but the CT revealed a small left haemothorax.

Small pleural collection example

CT scan revealing a small left haemothorax

No free fluid

If no free fluid is seen consider a repeat scan, perhaps 10 minutes later. If at any stage a black line anterior to the renal capsule, or black area in the pleural recess is present, the interpretation should be that there is free fluid present. Be cautious in making the assumption that this is blood – as previously referred to.

Once the skills of carrying out FAST have been acquired and free fluid can be visualised, it can be extended into these areas by choice, i.e. looking for free fluid in patients who may have a perforated peptic ulcer, or looking for free fluid in a female who may be suffering from a ruptured ectopic pregnancy.

Learning bite

Skills in determining if free fluid is present can be beneficial in various clinical settings.

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