What labs should be sent for thoracentesis (pleural fluid removal)?

Standard Laboratory Tests for Thoracentesis

For all thoracentesis procedures, pleural fluid should be sent for cytology, protein, LDH, pH, Gram stain, and microbiological culture to establish the diagnosis and guide management. 1

Core Laboratory Tests

Basic Tests (Send for All Patients)

• Appearance and odor - Note if fluid is serous, blood-tinged, frankly bloody, or purulent 1
• Protein - Essential for transudate/exudate differentiation 1
• Lactate dehydrogenase (LDH) - Required to apply Light's criteria for exudate/transudate classification 1, 2
• Cytology - Submit 25-50 mL (ideally 50 mL when possible) for optimal sensitivity 1
• Cell count and differential - Helps distinguish inflammatory from malignant processes 1, 2
• pH - Critical for identifying complicated parapneumonic effusions (pH < 7.2) 1, 2
• Gram stain - For rapid identification of bacterial pathogens 1
• Microbiological culture - Send in both sterile containers AND blood culture bottles to increase yield 1

Volume Requirements

• Cytology: 25-50 mL (minimum 25 mL, ideally 50 mL) 1
• Microbiology: 5-10 mL in blood culture bottles (both aerobic and anaerobic) 1
  • If limited volume: prioritize 2-5 mL for blood culture bottles over plain containers 1

Additional Tests Based on Clinical Suspicion

For Suspected Infection

• Pleural fluid glucose - Values <60 mg/dL suggest complicated parapneumonic effusion 2, 3
• Aerobic and anaerobic cultures - Use blood culture bottles to increase yield 1
• AFB stain and TB culture - If tuberculosis is suspected 1

For Suspected Malignancy

• Cytology with cell block preparation - Increases diagnostic yield 1
• Consider larger volume (50 mL) - Improves sensitivity for malignant cells 1, 4

For Suspected Tuberculosis

• Adenosine deaminase (ADA) - Levels >35 IU/L suggest TB in lymphocyte-predominant fluid 2
• Interferon gamma (IFN-γ) - Useful in high TB prevalence areas 1
• TB culture - Gold standard for diagnosis 1

For Suspected Hemothorax

• Hematocrit - If >50% of peripheral blood hematocrit, confirms hemothorax 5
  • Hematocrit <1% indicates blood is not significant 5

For Suspected Autoimmune Disease

• Antinuclear antibody (ANA) - Consider for suspected lupus pleuritis 1

Special Considerations

Processing Requirements

• Cytology samples should be processed by direct smear AND cell block preparation 1
• Microbiological samples should be sent in both white top containers AND blood culture bottles 1

Diagnostic Yield Considerations

• Pleural fluid cytology sensitivity varies by tumor type:
  • High yield (>90%): Breast cancer, pancreatic cancer 4
  • Low yield (<40%): Head and neck malignancy, sarcoma, renal cell cancer 4
• Consider early pleural biopsy for suspected mesothelioma or other malignancies with low cytological yield 1

Common Pitfalls to Avoid

1. Inadequate fluid volume - Collect sufficient volume (25-50 mL) for cytology 1
2. Improper collection technique - Use image guidance to reduce complications 1
3. Failure to inoculate blood culture bottles - Significantly reduces microbiological yield 1
4. Delayed pH analysis - Should be performed promptly on non-purulent effusions 1
5. Misinterpretation of bloody fluid - Not all bloody fluid is hemothorax; confirm with hematocrit 5

By following this systematic approach to pleural fluid analysis, clinicians can maximize diagnostic yield and guide appropriate management of patients with pleural effusions.