What tests are done for pleural fluid analysis in a patient presenting with pleural effusion?

Pleural Fluid Analysis Tests

All pleural fluid samples should be analyzed for protein, LDH, pH, cytology, Gram stain, acid-fast bacilli (AAFB) stain, and microbiological culture, with samples sent in both sterile containers and blood culture bottles to maximize diagnostic yield. 1, 2

Essential Initial Tests for All Pleural Effusions

Biochemical Analysis

• Protein and LDH: Measure to differentiate transudates from exudates using Light's criteria when protein is borderline (25-35 g/L) 1, 2
• pH measurement: Perform in all non-purulent effusions when infection is suspected; pH <7.2 indicates complicated parapneumonic effusion requiring drainage 1, 2
• Glucose: Measure in all exudates; levels <60 mg/dL suggest complicated parapneumonic effusion, empyema, rheumatoid pleurisy, or malignancy 3

Microbiological Studies

• Send 5-10 mL in both aerobic and anaerobic blood culture bottles PLUS sterile containers for Gram stain, AAFB stain, and TB culture 4, 1
• When volume is limited (2-5 mL), prioritize blood culture bottles over plain containers 4, 1

Cytological Examination

• Submit 25-50 mL for cytological analysis in suspected malignancy (strong recommendation by consensus) 4, 1
• Use both direct smear and cell block preparation 4, 1
• Cytology detects only 60% of malignant effusions; negative results should prompt further investigation 1, 3

Cell Count and Differential

• Obtain white blood cell count and differential on all exudative effusions 3, 5
• Lymphocyte predominance (>50%) suggests tuberculosis or malignancy 1

Specialized Tests Based on Clinical Context

For Suspected Tuberculosis

• Adenosine deaminase (ADA): Consider in high prevalence populations; levels >35-45 U/L with >50% lymphocytes suggest TB 1, 2, 3
• Use as exclusion test in low prevalence areas 1
• Tissue sampling via pleural biopsy is strongly recommended as the preferred diagnostic approach (69-97% sensitivity) for all suspected TB cases 1, 2

For Suspected Malignancy

• Cytology sensitivity varies by tumor type; direct biopsies should be considered for tumors with low cytological yield, especially mesothelioma 1
• Pleural fluid biomarkers should NOT be routinely used for diagnosing secondary pleural malignancy 4, 1
• If routine tests fail to diagnose suspected malignancy, thoracoscopy is advised 2, 5

For Suspected Heart Failure

• Serum NT-proBNP should be measured to support diagnosis in unilateral effusions suspected of heart failure 1
• Pleural fluid NT-proBNP is not superior to serum levels and should not be ordered routinely 1

For Suspected Rheumatoid Arthritis

• Measure pleural fluid glucose (≤1.6 mmol/L or 29 mg/dL strongly suggests RA), pH, and complement levels 6

Fluid Appearance Assessment

Visual Inspection

• Always record gross appearance and odor of pleural fluid: serous, blood-tinged, frankly bloody, or purulent 1, 2
• Turbid/milky fluid: Centrifuge to differentiate empyema (clear supernatant) from chylothorax (turbid supernatant due to lipids) 1, 2
• Hemorrhagic fluid: Measure hematocrit if hemothorax suspected; pleural fluid hematocrit >50% of peripheral blood confirms hemothorax 1

Procedural Recommendations

Thoracentesis Technique

• Image guidance (ultrasound) should ALWAYS be used to reduce complications (strong recommendation by consensus) 4, 1, 2
• Use fine-bore 21G needle with 50 mL syringe 1, 2
• Obtain 25-50 mL for optimal diagnostic yield, though 25 mL minimum is acceptable 1, 2

Diagnostic Algorithm

Step 1: Clinical Assessment

• Determine if transudate is likely based on clinical picture (heart failure, hypoalbuminemia, dialysis) 1, 2
• If transudate is clinically evident with confirmatory chest radiograph, thoracentesis is not needed unless atypical features exist or effusion fails to respond to treatment 2

Step 2: Classify Transudate vs Exudate

• Use protein levels and Light's criteria when borderline 1, 2, 5
• If pleural fluid meets exudative criteria but patient appears clinically to have transudate, measure serum-pleural fluid albumin gradient; if >1.2 g/dL, patient probably has transudate 5

Step 3: If Exudate Without Diagnosis

• Proceed to contrast-enhanced CT thorax 1
• Consider pleural biopsy (ultrasound/CT-guided, closed, or thoracoscopic) 1, 2

Critical Pitfalls to Avoid

• Do not perform blind thoracentesis without image guidance 2
• Diagnostic bronchoscopy is NOT indicated in undiagnosed effusion assessment unless the patient has hemoptysis or features suggestive of bronchial obstruction 2
• Light's criteria occasionally misidentify a transudate as an exudate; use serum-pleural fluid albumin gradient when clinical suspicion conflicts with laboratory results 5
• If small volume aspirate (<25 mL) has been non-diagnostic, send a larger volume if achievable, except when there is high suspicion of mesothelioma 4