What are the recommended orders for pleural fluid analysis in a patient presenting with a pleural effusion?

Pleural Fluid Analysis Orders

For any patient with pleural effusion requiring diagnostic thoracentesis, order the following core tests on all samples: 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. 1, 2

Procedural Approach

Always use image-guided thoracentesis (ultrasound or CT) to reduce complications—this is a strong consensus recommendation. 1, 2, 3 Use a fine-bore 21G needle with a 50 mL syringe for diagnostic sampling. 1, 3

Essential Tests for All Pleural Fluid Samples

Biochemical Analysis

• Protein and LDH: Measure to differentiate transudates from exudates using Light's criteria, particularly when protein is borderline (25-35 g/L). 1, 2, 4
• pH measurement: Perform in all non-purulent effusions when infection is suspected; pH ≤7.2 indicates complicated parapneumonic effusion requiring drainage. 1, 2
• Glucose: Obtain if infection suspected; <3.3 mmol/L (60 mg/dL) suggests complicated parapneumonic effusion when immediate pH unavailable. 1

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—this dual approach increases diagnostic yield. 1, 2
• If volume is limited (2-5 mL), prioritize blood culture bottles over plain sterile containers. 1, 2

Cytological Examination

• Submit 25-50 mL for cytological analysis in all cases where malignancy is suspected (strong consensus recommendation). 1, 2, 5
• Request both direct smear and cell block preparation. 1, 2
• Recognize that cytology detects only 60% of malignant effusions; negative results mandate further investigation. 1, 2

Visual Assessment (Document These Findings)

• Note appearance: serous, blood-tinged, frankly bloody, or purulent. 1, 2, 3
• Note odor: unpleasant aroma suggests anaerobic infection. 1
• If turbid/milky: centrifuge the sample—clear supernatant indicates empyema (cell debris), while persistently turbid supernatant indicates chylothorax (high lipid content). 1, 2, 3
• If hemorrhagic: measure pleural fluid hematocrit; >50% of peripheral blood hematocrit confirms hemothorax. 1, 2

Context-Specific Additional Tests

When Tuberculosis is Suspected

• Adenosine deaminase (ADA): Order in high TB prevalence populations; >35-45 U/L with >50% lymphocytes strongly suggests TB. 1, 2, 3, 6
• In low prevalence areas, use ADA as an exclusion test (high negative predictive value). 1, 2
• Interferon-gamma (IFN-γ): Consider ordering; >3.7 U/mL has 89% sensitivity and 97% specificity for TB. 1, 2, 6
• Critical caveat: Tissue sampling via pleural biopsy remains the preferred diagnostic approach for all suspected TB cases (69-97% sensitivity vs. fluid analysis alone). 1, 2, 3

When Malignancy is Suspected

• Cytology is the initial test, but diagnostic sensitivity varies dramatically by tumor type—mesothelioma has particularly low cytological yield. 1, 2
• Do NOT order pleural fluid biomarkers for diagnosing secondary pleural malignancy—they lack clinical utility. 1, 2
• If cytology is negative but suspicion remains high, proceed directly to pleural biopsy (thoracoscopic or image-guided) rather than repeating fluid cytology. 1, 2, 7

When Heart Failure is Suspected

• Order serum NT-proBNP (not pleural fluid NT-proBNP) to support diagnosis in unilateral effusions. 1, 2
• Pleural fluid NT-proBNP is not superior to serum levels and should not be routinely ordered. 1, 2

When Lupus Pleuritis is Suspected

• Pleural fluid antinuclear antibody (ANA): Order to support diagnosis. 1

When Complicated Parapneumonic Effusion is Suspected

• pH is the primary test: ≤7.2 mandates chest tube drainage if safe to insert. 1, 2
• If pH is 7.2-7.4 (intermediate risk): measure pleural fluid LDH—if >900 IU/L, strongly consider drainage, especially with fever, high fluid volume, low glucose (<4.0 mmol/L), pleural enhancement on CT, or septations on ultrasound. 1
• If pH ≥7.4: low risk, no immediate drainage indicated. 1

Critical Pitfalls to Avoid

• Do not contaminate pH samples with local anesthetic or heparin (expel all heparin from arterial blood gas syringe)—this falsely lowers pH. 1
• Avoid delays in pH measurement and eliminate air from sampling syringe—both falsely increase pH. 1
• Do not perform blind (non-image-guided) thoracentesis—this significantly increases pneumothorax risk. 1, 3, 5
• Do not send <25 mL for cytology unless unavoidable—sensitivity drops substantially with smaller volumes. 1, 2, 5
• Remember alternative diagnoses that mimic infected effusions with low pH: rheumatoid effusion and advanced malignancy/mesothelioma can also present with low pH and loculations. 1

Diagnostic Algorithm

1. Determine clinical likelihood of transudate (heart failure, hypoalbuminemia, dialysis) before thoracentesis. 1, 2, 3
2. If transudate is clinically obvious: treat underlying cause; only perform thoracentesis if atypical features present or no response to treatment. 1, 3
3. Perform image-guided thoracentesis: obtain 25-50 mL and send for all core tests listed above. 1, 2, 3, 5
4. Classify as transudate vs. exudate using protein/LDH and Light's criteria. 1, 2, 4
5. If exudate without diagnosis: proceed to contrast-enhanced CT thorax (with fluid still present for optimal pleural visualization) and consider pleural biopsy. 1, 2, 3