What pleural fluid tests should be ordered for a patient undergoing thoracentesis?

Pleural Fluid Tests to Order for Thoracentesis

For all patients undergoing thoracentesis, send pleural fluid for cell count with differential, protein, LDH, glucose, pH (if infection suspected), Gram stain, bacterial culture (including blood culture bottles), and cytology (25-50 mL for suspected malignancy). 1

Essential Tests for All Thoracenteses

Routine Diagnostic Panel

• Cell count with differential should be ordered to identify neutrophil-predominant (infection) versus lymphocyte-predominant (tuberculosis, malignancy) effusions 1, 2
• Protein and LDH are mandatory to apply Light's criteria for distinguishing transudates from exudates 2
• Glucose measurement helps identify complicated parapneumonic effusions (glucose <60 mg/dL indicates need for drainage) 2, 3

Microbiological Analysis

• Gram stain and bacterial culture must be sent in all cases where infection is remotely possible 1
• Blood culture bottles (aerobic and anaerobic) should receive 5-10 mL of pleural fluid when pleural infection is suspected, as this significantly increases diagnostic yield over plain containers alone 1
• If limited fluid volume (<5 mL available), prioritize 2-5 mL into blood culture bottles rather than plain containers 1

Diagnosis-Specific Testing

Suspected Malignancy

• Cytology requires 25-50 mL of pleural fluid for optimal diagnostic sensitivity (approximately 60-72% yield for malignant effusions) 1, 2
• Send at least 25 mL minimum; if less is obtained, send what you have but recognize the reduced sensitivity 1
• Samples should be processed by both direct smear and cell block preparation 1
• Important caveat: Mesothelioma has particularly low cytology yield, so consider proceeding directly to pleural biopsy if this diagnosis is suspected 1

Suspected Pleural Infection

• pH measurement is mandatory when infection is suspected and frank pus is not obtained; pH ≤7.2 indicates complicated parapneumonic effusion requiring chest tube drainage 1, 2, 3
• Send fluid in both plain white-top containers AND blood culture bottles (5-10 mL each for aerobic/anaerobic bottles) 1
• Glucose <60 mg/dL or pH <7.2 both indicate need for tube thoracostomy rather than antibiotics alone 2, 3

Suspected Tuberculosis

• Adenosine deaminase (ADA) should be ordered in high-prevalence populations; ADA >35-45 U/L strongly suggests tuberculous pleuritis in lymphocyte-predominant effusions 1, 2
• In low-prevalence populations, ADA can serve as an exclusion test (high negative predictive value) 1
• Mycobacterial culture should still be sent, as tissue sampling for culture and sensitivity is the preferred diagnostic method 1

Suspected Autoimmune Disease

• Pleural fluid ANA should be considered when lupus pleuritis is suspected 1
• Pleural fluid lymphocytosis mandates exclusion of tuberculosis and malignancy first 1

Tests NOT to Order Routinely

Limited Clinical Utility

• Pleural fluid NT-proBNP is not superior to serum NT-proBNP and should not be ordered routinely; use serum NT-proBNP instead for suspected heart failure 1
• Biochemical analysis (beyond protein, LDH, glucose, pH) is unnecessary in uncomplicated parapneumonic effusions 1
• Pleural fluid biomarkers (other than ADA) should not be used for diagnosing secondary pleural malignancy 1

Critical Technical Requirements

Image Guidance is Mandatory

• Ultrasound-guided thoracentesis must always be used to reduce pneumothorax risk from 50/1000 to 38/1000 procedures and increase success rate from 78% to 100% 1

Volume Requirements

• Minimum 25 mL, ideally 50 mL should be obtained for initial diagnostic thoracentesis to maximize yield 1, 4
• Ultrasound can detect as little as 20 mL of pleural fluid, making it ideal for guiding small-volume taps 4

Common Pitfalls to Avoid

• Never perform blind thoracentesis without ultrasound guidance—this significantly increases complication rates 1, 5
• Don't skip blood culture bottles for suspected infection—inoculating 5-10 mL into blood culture bottles dramatically improves bacterial recovery compared to plain containers alone 1
• Don't assume bilateral effusions are transudates—if heart size is normal or effusion is unilateral, perform thoracentesis 1
• Don't order pleural fluid NT-proBNP—serum levels are equally diagnostic and more cost-effective 1
• Don't send <25 mL for cytology unless unavoidable, as sensitivity drops substantially 1