Investigations

• A large effusion (>½ hemithorax) increases the risk of malignancy or TB. ^c  

Perform thoracentesis

• preferably using ultrasound guidance ^a  
• otherwise a 20G needle and syringe ^a; watch out for any subsequent pneumothorax ^c  

• biochemistry
  • protein ^c  
  • glucose ^b  
  • LDH  ^b  
  • pH  ^b
    
• cytology  ^c
• microscopy and culture of fluid 
  

If indicated, consider

• amylase (suspected pancreatitis or oesophageal rupture) ^c
• triglycerides ^c  (suspected chylous fluid)
  
• tumour markers 
  • carcinoembryonic antigen (CEA) ^c 
  • complement (C₃, C₄) ^c  
  • α1-antitrypsin ^c  
    
• antinuclear antibodies ^c  (suspected SLE)
• interferon- gamma (positive if > 140 pg/ml) ^c  (suspected TB)
• adenosine deaminase (positive if > 45 U/l) ^b  (suspected TB)

Consider taking the following blood tests:

• blood count ^d
• urea and electrolytes ^d
• glucose ^c
• protein ^b
• liver function tests ^b
• albumin ^b
• calcium ^d
• tumour markers ^c

If there is still no clear diagnosis, consider ^c  

• needle biopsy ^c   
• spiral CT, lung scan (or pulmonary angiography if neither available) ^d
• bronchoscopy  ^c  

followed by thoracoscopy ^c