How to Order Pleural Fluid Hematocrit
Send pleural fluid in a lavender/purple top (EDTA) tube to the laboratory with a request for hematocrit measurement, and simultaneously order a serum/peripheral blood hematocrit for comparison—hemothorax is diagnosed when the pleural fluid hematocrit is greater than 50% of the peripheral blood hematocrit.
Clinical Context and Diagnostic Criteria
Hemothorax is specifically defined as pleural fluid with a hematocrit exceeding 50% of the serum hematocrit value, occurring without natural or iatrogenic trauma to the lung or pleural space 1, 2, 3. This 50% threshold is the critical diagnostic criterion that distinguishes true hemothorax from bloody pleural effusions.
Practical Ordering Approach
Sample Collection Requirements
- Obtain adequate volume: While the 2023 British Thoracic Society guidelines don't specifically address hematocrit testing volumes, standard practice requires at least 3-5 mL of pleural fluid in an EDTA (anticoagulated) tube
- Simultaneous blood draw: Order a peripheral blood hematocrit at the same time to enable the diagnostic ratio calculation
- Timing: Process samples promptly to prevent clotting or hemolysis that could affect accuracy
Laboratory Requisition Specifics
When ordering, specify:
- "Pleural fluid hematocrit" (not just "pleural fluid analysis")
- Request concurrent serum/blood hematocrit for comparison
- Note clinical suspicion (e.g., "rule out hemothorax" or "bloody pleural effusion—quantify")
Clinical Scenarios Requiring Hematocrit Testing
You should specifically order pleural fluid hematocrit when:
- Grossly bloody pleural fluid is obtained during thoracentesis
- Patient is on anticoagulation (warfarin, DOACs like apixaban, heparin) and develops new pleural effusion 1
- Trauma (even minor) with pleural effusion
- Post-procedural effusion (post-thoracentesis, post-cardiac catheterization, post-oocyte retrieval 3)
- Unexplained pleural effusion in patients with coagulopathy or thrombocytopenia
- Rapid accumulation of pleural fluid with hemodynamic compromise
Integration with Standard Pleural Fluid Analysis
The 2023 BTS guidelines 4 recommend comprehensive pleural fluid testing but don't specifically mandate hematocrit in all cases. However, when bloody fluid is encountered, hematocrit becomes essential for:
- Distinguishing true hemothorax from traumatic tap
- Guiding management decisions (small-bore vs large-bore drainage, need for surgery)
- Identifying etiology (spontaneous vs traumatic vs iatrogenic)
Standard Pleural Fluid Orders Should Include:
- Cell count and differential
- Protein, LDH, glucose
- pH (if infection suspected) 4
- Cytology (25-50 mL for malignancy) 4
- Cultures (blood culture bottles for infection) 4
- Add hematocrit when fluid appears bloody
Common Pitfalls to Avoid
- Don't assume grossly bloody fluid equals hemothorax—traumatic taps can appear identical but have lower hematocrit ratios
- Don't delay ordering if fluid looks bloody—the 50% threshold is objective and clinically actionable
- Don't forget the comparison blood hematocrit—the ratio is what matters, not the absolute pleural fluid value
- Don't use clotted samples—anticoagulated tubes (EDTA) are required for accurate hematocrit measurement
Management Implications
Once hemothorax is confirmed (pleural fluid hematocrit >50% of blood):
- Drainage is mandatory: Use chest tube (12-French pigtail may be adequate for spontaneous hemothorax 1)
- Monitor output: Massive hemothorax (>1500 mL initial or >200 mL/hour) may require surgical intervention 2
- Address coagulopathy: Reverse anticoagulation if present
- Investigate cause: CT angiography if bleeding source unclear 2