What tests do we request for pleural effusion?

Diagnostic Tests for Pleural Effusion

For pleural effusion diagnosis, a diagnostic pleural fluid sample should be collected with a fine bore (21G) needle and analyzed for protein, LDH, pH, Gram stain, AAFB stain, cytology, and microbiological culture. 1

Initial Diagnostic Approach

• Pleural aspiration is the primary means of evaluating pleural fluid and should be performed for all new and unexplained pleural effusions 1, 2
• Ultrasound guidance should be used to confirm the presence of fluid and guide thoracentesis, reducing complications 2
• The sample should be placed in both sterile vials and blood culture bottles to increase diagnostic yield 1

Essential Laboratory Tests for All Pleural Fluid Samples

• Appearance and odor assessment: Note whether fluid is serous, blood-tinged, frankly bloody, or purulent 1
• Biochemical analysis:
  • Protein and LDH to differentiate transudate from exudate 1
  • If protein is between 25-35 g/l, Light's criteria should be used for accurate differentiation 1
  • pH measurement (especially important in non-purulent effusions if infection is suspected) 1
• Microbiological studies:
  • Gram stain and culture 1
  • AAFB (acid-alcohol fast bacilli) stain and TB culture 1
• Cytological examination: Essential for detecting malignant cells (detects approximately 60% of malignant effusions) 1
• Cell count with differential 3

Additional Tests for Specific Clinical Scenarios

• For suspected hemorrhagic effusion: Pleural fluid hematocrit to diagnose haemothorax 1
• For turbid/milky fluid: Centrifuge the sample - if supernatant remains turbid, test for lipids to diagnose chylothorax 1
• For suspected tuberculous effusion: Adenosine deaminase (ADA) testing - 91% sensitivity and 88% specificity for TB pleurisy 4
• For suspected empyema or complicated parapneumonic effusion: pH <7.2 indicates need for drainage 5, 3

Diagnostic Algorithm

1. Assess if clinical picture suggests transudate (heart failure, hypoalbuminemia, dialysis) 1

   • If yes, treat the underlying cause
   • If no, proceed to pleural aspiration

2. Perform pleural aspiration and send for standard tests 1

3. Evaluate for specific conditions:

   • Empyema: Purulent fluid, positive cultures, pH <7.2 2
   • Chylothorax: Milky appearance, high lipid content 1
   • Haemothorax: Bloody fluid, high hematocrit 1

4. Determine if transudate or exudate:

   • If transudate: Treat underlying cause (heart failure, cirrhosis, etc.) 2
   • If exudate: Further investigation needed 2

5. If diagnosis remains unclear after initial testing:

   • Request contrast-enhanced CT thorax 1, 2
   • Consider pleural tissue biopsy (ultrasound/CT guided, closed pleural biopsy, or thoracoscopy) 1, 2

Common Pitfalls and Caveats

• Pulmonary embolism can cause pleural effusion but tests on pleural fluid are often unhelpful for diagnosis - maintain high index of suspicion 1
• Only 60% of malignant effusions are diagnosed by cytological examination alone; CT scan and pleural biopsy may be necessary 1
• When sending specimens for microbiological examination, use both sterile tubes and blood culture bottles to increase yield 1
• ADA levels can be elevated in conditions other than TB (empyema, rheumatoid pleurisy), reducing specificity in low TB prevalence regions 4
• Pleural fluid pH should be measured with a blood gas analyzer (not litmus paper) and transported anaerobically on ice for accurate results 1