Transudate vs. Exudate: What's the Difference?

Tests to determine the source of fluid effusions accumulating in body cavities (exudative vs. transudative) are important for diagnosis of a range of conditions.

Light’s criteria is a calculation that helps determine whether fluid within a body cavity, called an effusion, is caused by transudate or exudate. Using the patient’s blood and the fluid in the effusion and comparing specific characteristic parameters, medical practitioners can determine if the fluid is caused by internal pressure problems (transudate) or cellular leakage (exudate).

“Transudate” is fluid buildup caused by systemic conditions that alter the pressure in blood vessels, causing fluid to leave the vascular system. 

“Exudate” is fluid buildup caused by tissue leakage due to inflammation or local cellular damage. 

The fluid is exudate if one of the following Light’s criteria is present:  

• Effusion protein/serum protein ratio greater than 0.5
• Effusion lactate dehydrogenase (LDH)/serum LDH ratio greater than 0.6
• Effusion LDH level greater than two-thirds the upper limit of the laboratory's reference range of serum LDH

Lactate dehydrogenase (LDH) is an enzyme normally inside of cells in the body. When there is damage to cells, LDH leaks out and becomes part of the effusion. Measuring a high LDH in the effusion is indicative of cell damage, which typically comes from an exudative process. This is why one of Light’s criteria measures LDH in the effusion.

• Partially collapsed lung tissue (atelectasis): Due to increased negative pressure inside the lung cavity
• Cerebrospinal fluid (CSF) leak into lung cavity (pleural space): Thoracic spine injury, ventriculoperitoneal (VP) shunt dysfunction
• Heart failure
• Liver dysfunction
• Low blood albumin (hypoalbuminemia)
• Iatrogenic (misplaced catheter into lung)
• Nephrotic syndrome
• Peritoneal dialysis
• Blockage of urinary system causing urine backup in the body (urinothorax due to obstructive uropathy)

• Abdominal fluid: Abscess in tissues near lung, fluid in the abdomen (ascites), Meigs syndrome, pancreatitis
• Connective-tissue disease: Churg-Strauss disease, lupus, rheumatoid arthritis, Wegener granulomatosis
• Endocrine: Low thyroid (hypothyroidism), ovarian hyperstimulation
• Iatrogenic: Drug-induced, esophageal perforation, feeding tube in lung
• Infectious: Abscess in tissues near lung, bacterial pneumonia, fungal disease, parasites, tuberculosis
• Inflammatory: Acute respiratory distress syndrome (ARDS), asbestosis, pancreatitis, radiation, sarcoidosis, high levels of urea in the blood (uremia)
• Lymphatic abnormalities: Chylothorax (fluid around the lungs), cancer in the lymph nodes, lymphangiectasia (over-dilation of lymph vessels)
• Malignancy: Cancer, lymphoma, leukemia, mesothelioma, paraproteinemia