Hemothorax
The diagnosis of hemothorax is made based on pleural fluid Hct. The standard cutoff of 50% of blood Hct is based on the risk of having a complex pleural space, requiring chest tube or more aggressive management.
-from Light’s Pleural Diseases
It takes very little blood to both make fluid look grossly bloody by visual inspection and on ultrasound. This patient had pleural fluid Hct < 2%.
This patient actually has a hemothorax with hematocrit sign.
Blood accumulated in any serous cavity relatively slowly becomes defibriginated, as the serous surface gets coated with fibrin (Broadie1981). Consequently, if during a procedure a tap returns bloody fluid, don’t panic quite yet. Squirt it out of the syringe on to a gauze. If it does not clot, you did NOT just enter a blood vessel, and can breath easy.
Similarly, blood in body cavities usually does not appear clotted on US, unless it accumulates very rapidly or there is a lot of it.
Here is an example of a blood clot in the pleural cavity. This is an EMERGENCY!