Pleural Fluid Analysis

The pleural space normally holds 5 to 15 mL of fluid, filtered from parietal pleural capillaries and drained by parietal lymphatics. A pleural effusion forms when filtration increases (heart failure, hypoalbuminaemia, inflammation) or lymphatic drainage falls (malignancy, obstruction). Thoracentesis (pleural tap) removes fluid for analysis, usually 30 to 60 mL, using ultrasound. The first question is whether the fluid is a transudate or an exudate, and the second is the specific cause.

The test determines the cause of an effusion, directs drainage in infection and malignancy, and prevents unnecessary investigation when a transudate has an obvious cause. Pleural effusions affect over 1.5 million people a year in the United States, and the leading causes are heart failure, pneumonia (parapneumonic effusion), malignancy and tuberculosis in endemic regions.

Malignant effusion carries a median survival of 3 to 12 months. Parapneumonic effusions complicate 20 to 40% of pneumonias, and 10% need drainage. Tuberculous pleurisy is the commonest form of extrapulmonary tuberculosis in many African and Asian countries.

Indications. New unexplained pleural effusion, effusion with fever (suspected empyema), suspected malignancy, suspected tuberculosis, effusion not responding to treatment of heart failure, and effusion with an atypical presentation (unilateral transudate, asymmetric bilateral effusions, normal heart size). A clear heart failure effusion that responds to diuretics may not need a tap.

Specimen. Fluid in a sterile syringe, divided into:

• plain tube or heparin for chemistry (protein, LDH, glucose, pH, amylase, triglycerides)
• EDTA tube for cell count and differential
• sterile pots for culture and mycobacterial testing
• an EDTA or heparin sample or inoculated blood culture bottles (increasing yield)

Collect a heparinised syringe, kept on ice, for pH if infection is suspected (do not use lidocaine in the syringe, since acidic lidocaine lowers pH). Send paired serum for protein, LDH and glucose taken at the same time. Cytology needs at least 50 to 100 mL for best yield.

Appearance. Straw coloured clear fluid is typical of transudate. Turbid or purulent: empyema. Bloody: malignancy, trauma, pulmonary embolism, tuberculosis (red cells over 100 000 per microL suggests these). Milky: chylothorax (triglycerides over 1.24 mmol/L, or 110 mg/dL) or pseudochylothorax (cholesterol crystals). Food particles: oesophageal rupture. Brown fluid: long standing bleeding or amoebic liver abscess.

Light's criteria. An effusion is an exudate if one or more of the following is present:

• Pleural fluid protein to serum protein ratio over 0.5.
• Pleural fluid LDH to serum LDH ratio over 0.6.
• Pleural fluid LDH over two thirds of the upper limit of normal serum LDH.

Sensitivity for exudate is about 98%, with specificity of about 80%. Up to 25% of transudates in patients on diuretics are misclassified as exudates. If a diuresed patient has a borderline result, the serum to pleural fluid albumin gradient over 12 g/L (1.2 g/dL) supports a transudate, and pleural fluid NT-proBNP over 1500 ng/L supports heart failure.

Transudate causes. Heart failure (the commonest), cirrhosis (hepatic hydrothorax), nephrotic syndrome, hypoalbuminaemia, peritoneal dialysis, hypothyroidism, urinary obstruction, superior vena cava obstruction, and early atelectasis.

Exudate causes. Parapneumonic effusion and empyema, malignancy (lung, breast, lymphoma, mesothelioma, ovarian), tuberculosis, pulmonary embolism, connective tissue diseases (rheumatoid arthritis, lupus), pancreatitis, oesophageal rupture, post-cardiac injury syndrome, asbestos related effusion, drug induced effusion (amiodarone, nitrofurantoin, methotrexate), chylothorax, haemothorax, and uraemia.

Specific tests.

• pH: under 7.20 in a parapneumonic effusion indicates complicated effusion and the need for chest drainage. Low pH is also found in rheumatoid pleurisy, malignancy, tuberculosis and oesophageal rupture. If pH is not available, glucose under 2.2 mmol/L (40 mg/dL) serves.
• Glucose: under 3.3 mmol/L (60 mg/dL) in empyema, rheumatoid effusion (often under 1.7 mmol/L), malignancy, tuberculosis and lupus.
• LDH over 1000 U/L: empyema, rheumatoid pleurisy, malignancy (high), and paragonimiasis.
• White cells: over 10 000 per microL polymorphs suggests parapneumonic effusion. Lymphocytes over 50% of cells suggest tuberculosis, malignancy, lymphoma, rheumatoid arthritis and chylothorax. Eosinophils over 10% suggest air or blood in the pleural space, drug reaction, asbestos effusion, parasitic infection and malignancy.
• Adenosine deaminase: over 40 U/L has sensitivity of 90 to 95% and specificity of 85 to 90% for tuberculous pleurisy, and a value under 30 U/L with a lymphocyte exudate makes it unlikely. Interferon gamma and mycobacterial culture add confirmation. Pleural biopsy gives granulomas and culture in 80 to 90%.
• Cytology: first sample diagnostic in about 60%, 70 to 80% with a second sample, and a third adds little. Pleural biopsy (radiologically targeted or thoracoscopic) raises the yield to over 90%.
• Amylase: over 200 U/L or higher than serum suggests pancreatic effusion, oesophageal rupture (salivary type) or malignancy.
• Triglycerides over 1.24 mmol/L (110 mg/dL): chylothorax. Under 0.56 mmol/L (50 mg/dL): excludes it.
• Haematocrit: pleural fluid to blood haematocrit ratio over 0.5 defines haemothorax.
• Rheumatoid factor and complement are not diagnostic. ANA titre over 1:160 supports lupus pleuritis.
• Gram stain and culture, with blood culture bottle inoculation, increase yield in empyema.
• Mesothelin and other markers are not diagnostic alone. CEA over 10 microg/L in the fluid supports adenocarcinoma.

Critical values and findings needing immediate action.

• Pus, positive Gram stain or culture, pH under 7.20, or glucose under 2.2 mmol/L: chest tube drainage and antibiotics without delay.
• Bloody effusion after trauma or with haemodynamic compromise: haemothorax, and a chest drain.
• Oesophageal rupture (high amylase, low pH, food particles): urgent surgical review.
• Tension effects (mediastinal shift) or respiratory distress from a large effusion: therapeutic drainage.
• Malignant cells: oncology referral and consideration of an indwelling catheter or pleurodesis.
• Lymphocytic exudate with adenosine deaminase over 40 U/L in an endemic region: start tuberculosis treatment pending cultures.

Interferences and pitfalls.

• Light's criteria mislabel up to 25% of transudates as exudates after diuresis, and the albumin gradient or NT-proBNP corrects this.
• Pleural fluid pH needs an anaerobic heparinised syringe processed in a blood gas analyser. Lidocaine or air in the sample gives a falsely high or low pH.
• Cell count is affected by a traumatic tap, and blood stained fluid needs correction.
• A negative cytology does not exclude malignancy, and repeat or biopsy is needed.
• Culture is negative in up to 40% of empyema, and molecular tests may help.
• Adenosine deaminase is falsely negative in some HIV patients and falsely positive in empyema, lymphoma and rheumatoid effusion.
• Tuberculous pleurisy culture yield is only 25 to 40%, and pleural biopsy is better.
• Thoracentesis complications: pneumothorax (about 5%, less with ultrasound), bleeding, re-expansion pulmonary oedema (removal over 1.5 L at once is avoided) and vasovagal reactions.
• Coagulopathy and anticoagulants increase bleeding risk, though taps done with ultrasound are generally safe.

Special groups.

• Children: parapneumonic effusion is common, with Streptococcus pneumoniae, Staphylococcus aureus and Streptococcus pyogenes, and drainage is chosen by size and pH.
• Pregnancy: effusions are uncommon, and thoracentesis is safe with ultrasound.
• Older adults: heart failure and malignancy predominate.
• Immunocompromised patients: tuberculosis, fungal infection and lymphoma.
• Regions: tuberculous effusion is common in young adults and HIV patients, and empyema often presents late.

Follow up tests. Chest radiograph, thoracic ultrasound, contrast CT of chest, NT-proBNP, serum albumin, liver and kidney function, sputum culture and tuberculosis tests (Xpert, culture), HIV test, ANA and rheumatoid factor, pleural biopsy, thoracoscopy, D-dimer and CT pulmonary angiography for embolism, and echocardiography.