What are the different types of exudate?

Types of Exudate

Exudate appearance can be classified into four main categories: serous (clear), blood-tinged, frankly bloody, and purulent, with additional specialized types including turbid/milky fluids that require centrifugation to distinguish between empyema, chylothorax, and pseudochylothorax. 1

Primary Classification by Appearance

The British Thoracic Society guidelines establish a systematic approach to categorizing exudate based on visual inspection:

Serous Exudate

• Clear, straw-colored fluid that typically indicates a less inflammatory process 1
• Most commonly seen in early inflammatory responses or certain malignancies 1

Blood-Tinged Exudate

• Fluid with visible blood but not grossly hemorrhagic 1
• Requires differentiation from true hemothorax using hematocrit measurement 1

Frankly Bloody (Hemorrhagic) Exudate

• Grossly bloody fluid requiring hematocrit assessment 1
• If pleural fluid hematocrit is >50% of peripheral blood hematocrit, this confirms hemothorax 1
• If hematocrit is <1%, the blood is not clinically significant 1
• Common causes include malignancy, pulmonary embolism with infarction, trauma, benign asbestos pleural effusions, and post-cardiac injury syndrome 1, 2

Purulent Exudate

• Thick, opaque fluid indicating infection 1
• The presence of an unpleasant odor suggests anaerobic infection and should guide antibiotic selection 1

Specialized Exudate Types Requiring Further Testing

Turbid/Milky Exudate

When fluid appears turbid or milky, centrifugation is mandatory to determine the underlying cause 1:

• If supernatant clears after centrifugation: The turbidity was due to cell debris, indicating empyema is likely 1
• If supernatant remains turbid after centrifugation: High lipid content is present, suggesting chylothorax or pseudochylothorax 1, 3

Chylothorax

• Milky appearance due to high lipid content that persists after centrifugation 1, 3
• Caused by lymphatic disruption with triglyceride-rich fluid 3

Pseudochylothorax

• Turbid, milky appearance that remains after centrifugation due to cholesterol crystals rather than triglycerides 3
• Typically occurs in chronic, long-standing effusions 3

Biochemical Classification: Transudate vs. Exudate

Beyond visual appearance, exudates are biochemically distinguished from transudates:

Exudative Characteristics

• Protein >30-35 g/L (when serum protein is normal) 1, 2
• Light's criteria should be applied when protein is 25-35 g/L: pleural fluid/serum protein ratio >0.5, pleural fluid/serum LDH ratio >0.6, or pleural fluid LDH >0.67 of upper limit of normal 1, 2, 3
• pH <7.2 indicates complicated exudate (empyema, rheumatoid arthritis, esophageal rupture, or malignancy) 1
• Glucose <3.3 mmol/L suggests empyema, rheumatoid disease, lupus, tuberculosis, malignancy, or esophageal rupture 1

Transudative Characteristics

• Protein <25 g/L (when serum protein is normal) 1, 2
• Typically clear and serous in appearance 2
• Most commonly due to heart failure (>80% of cases) 3

Clinical Algorithm for Exudate Assessment

Step 1: Note appearance and odor immediately upon aspiration 1

Step 2: If bloody, measure pleural fluid hematocrit to confirm or exclude hemothorax 1

Step 3: If turbid/milky, centrifuge the specimen 1:

• Clear supernatant → empyema likely
• Turbid supernatant → chylothorax or pseudochylothorax

Step 4: Measure protein, LDH, pH, and glucose for all non-purulent effusions 1, 2

Step 5: Send for Gram stain, culture (including blood culture bottles), AAFB stain, and cytology 1

Common Pitfalls to Avoid

• Do not assume all bloody effusions are malignant—pulmonary embolism, trauma, and benign asbestos effusions also cause bloody fluid 2
• Do not rely on imaging alone (ultrasound echogenicity has only 63% specificity for transudates) to determine exudate type—biochemical analysis is mandatory 1, 3
• Do not skip centrifugation for turbid/milky fluids, as this is the only way to distinguish empyema from chylothorax 1
• Always note odor, as anaerobic infection has a characteristic unpleasant smell that guides antibiotic choice 1
• Measure pH in all non-purulent effusions when infection is suspected—pH <7.2 has better discriminatory value than glucose for pleural infection 1