What tests should be ordered on pleural fluid for comprehensive diagnostic evaluation?

Pleural Fluid Analysis: What to Send

For comprehensive diagnostic evaluation, send pleural fluid for protein, LDH, pH, cytology, Gram stain, acid-fast bacilli (AAFB) stain, and microbiological culture, placing samples in both sterile containers and blood culture bottles. 1, 2

Core Laboratory Tests (Send on Every Sample)

Biochemical Analysis

• Protein: Essential for transudate/exudate differentiation 1, 2
• Lactate dehydrogenase (LDH): Required to apply Light's criteria when protein is borderline (25-35 g/L) 1, 2
• pH: Mandatory in all non-purulent effusions when infection is suspected; pH <7.2 indicates complicated parapneumonic effusion requiring drainage 1, 2

Microbiological Studies

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

Cytological Examination

• Submit 25-50 mL for cytological analysis in suspected malignancy 2
• Use both direct smear and cell block preparation 2
• Recognize that cytology detects only 60% of malignant effusions; negative results should prompt pleural biopsy 1, 2

Visual Assessment (Document at Bedside)

• Note appearance: serous, blood-tinged, frankly bloody, or purulent 1
• Note odor: unpleasant aroma suggests anaerobic infection 1
• If turbid/milky: centrifuge the sample—clear supernatant indicates empyema (cell debris), turbid supernatant indicates chylothorax (high lipid content) 1
• If bloody: measure pleural fluid hematocrit; >50% of peripheral blood hematocrit confirms hemothorax 1

Context-Specific Additional Tests

For Suspected Tuberculosis

• Adenosine deaminase (ADA): In high TB prevalence populations, ADA >35-45 U/L with >50% lymphocytes suggests TB 2
• Interferon-gamma (IFN-γ): 89% sensitivity and 97% specificity for TB 2
• Tissue sampling is strongly preferred over fluid analysis alone (69-97% sensitivity) 2

For Suspected Heart Failure

• Serum NT-proBNP (not pleural fluid NT-proBNP) should be measured to support diagnosis in unilateral effusions 2
• Pleural fluid NT-proBNP is not superior to serum levels and should not be ordered 2

For Suspected Malignancy

• Do NOT routinely order pleural fluid biomarkers for diagnosing secondary pleural malignancy 2
• Cytology sensitivity varies by tumor type; mesothelioma has particularly low cytological yield and requires direct tissue biopsy 2

Critical Procedural Points

• Use fine-bore 21-gauge needle with 50 mL syringe for diagnostic sampling 1
• Always use ultrasound guidance to reduce complications 2, 3
• Split the sample appropriately: blood culture bottles for microbiology, sterile containers for stains and TB culture, adequate volume for cytology 1, 2

Common Pitfalls to Avoid

• Do not skip pH measurement in suspected parapneumonic effusions—this single test determines need for drainage 1, 2
• Do not send only sterile containers for microbiology; blood culture bottles significantly increase diagnostic yield 1, 2
• Do not order pleural fluid analysis on bilateral effusions that clearly suggest transudate (heart failure, hypoalbuminemia, dialysis) unless atypical features are present or there is no response to treatment 1, 3
• Do not assume pleural fluid tests diagnose pulmonary embolism—maintain high clinical suspicion as fluid analysis is unhelpful for this diagnosis 1, 3
• Do not rely solely on pleural fluid protein if serum protein is abnormal or if pleural protein is 25-35 g/L; apply Light's criteria in these cases 1, 2