Do a pleural tap and compare with serum protein and LDH levels (Light's criteria)
It is exudate if any one of below criteria positive:
Effusion protein / serum protein >0.5
Effusion LDH / serum LDH >0.6
Effusion LDH > 2/3 upper limit of serum LDH
It is exudate if any one of below criteria positive:
Effusion protein / serum protein >0.5
Effusion LDH / serum LDH >0.6
Effusion LDH > 2/3 upper limit of serum LDH