HEMOTHORAX
The pleural fluid hematocrit is >50% of the simultaneous peripheral blood hematocrit.
If the pleural fluid hematocrit value is not available , one can estimate the pleural fluid hematocrit by dividing the pleural fluid RBC count by 100,000.
If the pleural fluid appears bloody and the hematocrit is <50%, it is considered a hemorrhagic effusion but not a true hemothorax
Ref: Fishman 6th ed