What labs should be ordered for a patient with loculated pleural effusion or empyema?

Laboratory Tests for Loculated Pleural Effusion or Empyema

For patients with loculated pleural effusion or empyema, diagnostic thoracentesis with analysis of pleural fluid for protein, LDH, pH, glucose, Gram stain, culture (in blood culture bottles), cell count with differential, and cytology is essential for proper management and should be performed in all cases. 1

Initial Diagnostic Approach

Imaging Studies (Prior to Laboratory Testing)

• Chest radiograph (posteroanterior view) - first-line imaging to detect pleural fluid 1, 2
• Ultrasound scan of chest - mandatory to:
  • Confirm presence of pleural fluid
  • Differentiate free from loculated fluid
  • Guide thoracentesis or drain placement
  • Estimate size of effusion 1, 2

Essential Laboratory Tests on Pleural Fluid

1. Biochemical Analysis:

   • Protein level (to distinguish transudate from exudate)
   • LDH (lactate dehydrogenase) - particularly important for borderline protein values
   • pH - critical for determining need for drainage (pH <7.20 indicates need for chest tube)
   • Glucose - levels <60 mg/dL (3.4 mmol/L) suggest complicated parapneumonic effusion

2. Microbiological Studies:

   • Gram stain
   • Culture (send in both sterile tubes AND blood culture bottles to increase yield)
   • AAFB (acid-alcohol fast bacilli) stain and TB culture

3. Cytological Examination:

   • Cell count with differential
   • Cytology for malignant cells

4. Blood Tests:

   • Full blood count (for anemia, white count with differential, platelet count)
   • Blood cultures (including anaerobic bottle)
   • C-reactive protein (useful marker of progress)
   • Electrolytes (to detect inappropriate ADH syndrome)
   • Serum albumin (often low in effusions) 1

Special Considerations for Specific Conditions

For Suspected Empyema

• Note appearance and odor of fluid (purulent appearance or unpleasant odor suggests anaerobic infection)
• If fluid is turbid/milky, centrifuge it:
  • Clear supernatant indicates cell debris (likely empyema)
  • Turbid supernatant suggests chylothorax 1

For Suspected Chylothorax

• Triglyceride levels (>1.24 mmol/L or 110 mg/dL suggests chylothorax)
• Cholesterol levels
• Examination for chylomicrons and cholesterol crystals 1

For Suspected Tuberculous Effusion

• AAFB stain (sensitivity only 10-20%)
• TB culture (sensitivity 25-50%)
• Consider pleural biopsy for histology and culture (improves diagnostic rate to ~90%) 1

Management Algorithm Based on Laboratory Results

1. If pH <7.20, glucose <60 mg/dL, or positive Gram stain/culture:

   • Chest tube drainage is indicated
   • Consider intrapleural fibrinolytics for loculated collections 3, 4

2. If fluid recurs after initial thoracentesis:

   • If patient is clinically stable AND glucose >60 mg/dL, pH >7.2, LDH <3x upper normal limit, and negative cultures: observe
   • If any criteria not met: repeat thoracentesis with diagnostic evaluation 5

3. If loculations persist despite drainage:

   • Consider intrapleural fibrinolytic therapy (74% response rate across different types of loculated collections) 6
   • If drainage remains inadequate, consider thoracoscopy or surgical intervention 5

Common Pitfalls to Avoid

1. Failure to send pleural fluid in blood culture bottles - this significantly reduces the diagnostic yield for infections 1

2. Relying solely on pleural fluid cytology for malignancy diagnosis - only 60% of malignant effusions are diagnosed by cytology alone 1

3. Not measuring pH properly - must be collected anaerobically in a heparinized syringe and analyzed promptly on a blood gas analyzer 1

4. Missing tuberculosis - smears and cultures have limited sensitivity; consider pleural biopsy when TB is suspected 1

5. Performing unnecessary CT scans - ultrasound is sufficient for most cases and avoids radiation exposure 1

By following this systematic approach to laboratory testing, the etiology of loculated pleural effusions and empyema can be determined in the majority of cases, allowing for appropriate and timely management.