Ok, so classically we talk about transudates vs exudates by pleural fluid analysis. I’ll reframe this (I’m sure it’s already obvious to you, but hey) as ‘how can we diagnose an exudate, and the cause’

Protein & LDH

1. Is it an exudate? Light’s criteria
   PROTEIN: Pleural at least 50% of serum level, or
   LDH: Pleural at least 60% of serum, or
   LDH (IF serum high): Pleural at least 2/3 of max serum reference range
   
   Any of the above? YAY! It’s an exudate!
   equivocal? –> ALBUMIN (if serum – pleural <1.2, exudate)
   
2. Further testing on the PLEURAL FLUID
   • Differential
   • Gram stain, culture & cytology
   • Glucose
   • Serial LDH testing (for cessation vs continuation of process)
   • pH
   • Amylase
   • Cholesterol

Differential

• Lymphocytes – Lymphoid cells! T- / B-cells
  • Cancer
  • TB pleuritis
• Neutrophils – PUS stuff!
  • Pneumonia
  • Empyema
  • Pancreatitis
  • Pulmonary embolism
• Eosinophils – Allergic/irritant stuff
  • Pneumothorax
  • Haemothorax
  • Asbestosis
  • Eosinophilic Granulomatosis with Polyangitis
• Mononuclear – Chronic inflammatory stuff
  • Chronic inflammatory disease

Glucose

• LOW in infection & malignancy
• rarely: low in TB, HTx, EGPA

pH

• <7.2 needs drainage. Implications are:
  • <7.2 & pneumonia –> drain
  • <7.2 & malignancy –> 30 days to live… more or less.

Amylase

• Pancreatitis

Cholesterol

• Chylothorax