What tests should be ordered for a full analysis of pleural fluid?

Comprehensive Pleural Fluid Analysis

For a complete pleural fluid analysis, you should collect samples for protein, LDH, cytology, pH, glucose, Gram stain, culture (in both sterile containers and blood culture bottles), cell count with differential, and specific biomarkers based on clinical suspicion. 1

Core Testing Requirements

Sample Collection

• Use image-guided thoracentesis to reduce complications 1
• Collect with a fine bore (21G) needle and a 50 ml syringe 1
• For cytology: 25-50 mL of fluid (minimum 25 mL, ideally 50 mL) 1
• For microbiology: Send samples in both sterile containers AND blood culture bottles 1
  • If limited volume: prioritize 5-10 mL for blood culture bottles (both aerobic and anaerobic) 1

Essential Tests for All Pleural Fluid Samples

1. Appearance and odor assessment 1

   • Note if serous, blood-tinged, frankly bloody, or purulent
   • Unpleasant odor may indicate anaerobic infection

2. Biochemical analysis

   • Protein (pleural fluid and serum for comparison) 1
   • LDH (pleural fluid and serum for comparison) 1
   • pH (especially important for suspected infection) 1
   • Glucose 1

3. Cytology

   • Process samples by direct smear and cell block preparation 1
   • Note: Only detects ~60% of malignant effusions 1

4. Cell count with differential 1

   • Lymphocyte predominance may suggest TB or malignancy
   • Neutrophil predominance suggests acute infection

5. Microbiology

   • Gram stain 1
   • Culture (bacterial) 1
   • AAFB (acid-alcohol fast bacilli) stain for TB 1

Additional Tests Based on Clinical Suspicion

1. If infection suspected:

   • pH measurement (pH <7.2 indicates complicated parapneumonic effusion) 1, 2
   • Cultures in both sterile containers and blood culture bottles 1

2. If tuberculosis suspected:

   • Adenosine deaminase (ADA) - levels >35 IU/L suggest TB in lymphocyte-predominant fluid 1, 2
   • Interferon gamma (IFN-gamma) testing 1
   • TB culture 1

3. If malignancy suspected:

   • Cytology (send larger volumes - at least 50 mL if possible) 1
   • Consider tumor markers based on suspected primary malignancy

4. If chylothorax suspected:

   • Triglycerides 3
   • Cholesterol 3
   • If turbid/milky: centrifuge to differentiate chylothorax from empyema 1

5. If hemothorax suspected:

   • Hematocrit (if >50% of peripheral blood hematocrit, confirms hemothorax) 1

6. If autoimmune disease suspected:

   • Antinuclear antibody (ANA) - useful for diagnosing lupus pleuritis 1

7. If pancreatitis or esophageal rupture suspected:

   • Amylase 1
   • Consider isoenzyme analysis to differentiate salivary from pancreatic origin 1

8. If heart failure suspected:

   • Consider NT-proBNP (serum test preferred over pleural fluid) 1

Interpretation Framework

1. First step: Transudate vs. Exudate

   • Apply Light's criteria 1:
     • Pleural fluid protein/serum protein >0.5
     • Pleural fluid LDH/serum LDH >0.6
     • Pleural fluid LDH >2/3 upper limit of normal for serum LDH

2. If exudate, further categorize based on:

   • pH and glucose levels (low in complicated parapneumonic effusions, rheumatoid disease, TB, malignancy) 1
   • Cell count differential (lymphocyte vs. neutrophil predominance) 1
   • Cytology results 1
   • Specific biomarkers (ADA, amylase, etc.) 1, 2

Common Pitfalls to Avoid

1. Inadequate sample volume

   • Collect sufficient fluid (at least 25-50 mL for cytology) 1
   • Small volumes reduce diagnostic yield, especially for malignancy 1

2. Improper collection technique

   • Always use image guidance to reduce complications 1
   • Avoid blind pleural biopsies 1

3. Delayed processing

   • pH should be measured immediately (using blood gas analyzer) 1
   • Transport specimens promptly to laboratory

4. Relying solely on pleural fluid cytology for malignancy diagnosis

   • Remember cytology has only ~60% sensitivity for malignancy 1
   • Consider pleural biopsy if high suspicion despite negative cytology 1

5. Misinterpreting transudates with borderline values

   • If protein is between 25-35 g/L, use complete Light's criteria 1
   • Consider serum-pleural fluid albumin gradient if clinical picture suggests transudate but Light's criteria indicate exudate 4

By following this comprehensive approach to pleural fluid analysis, you can maximize diagnostic yield and guide appropriate management decisions for patients with pleural effusions.