Exudates are characterised by protein levels of > 25 g/L; commonly associated with malignancy, pneumonia, tuberculosis.
Transudates have protein levels of < 25 g/L and are seen in congestive Cardiac failure, Cirrhosis, Nephrotic syndrome, Hypothyroidism, Meigs syndrome.
Lactate dehydrogenase isoenzymes in exudates may help identify the likely source:
LD1 and LD2 – red cells
LD2 and LD3 – malignancy
LD5 – neutrophils.
Neutrophils are increased in bacterial infection; lymphocytes may be increased in Tuberculosis, malignancy, including lymphoma.
Glucose levels may be low in pleuritis or pericarditis due to Rheumatoid arthritis.
Amylase is raised in pleuritis associated with Pancreatitis.
Cytology may detect malignant cells from mesothelioma, metastatic or locally invasive tumours, as well as reactive/inflammatory changes.