Light's Criteria

Determination of transudate versus exudate source of pleural effusion

Fluid is exudate if one of the following Light’s criteria is present[1, 2, 3, 4] :

• 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

Exudative effusions

Causes of exudative effusions include the following:

• Abdominal fluid: Abscess in tissues near lung, ascites, Meigs syndrome, pancreatitis

• Connective-tissue disease: Churg-Strauss disease, lupus, rheumatoid arthritis, Wegener granulomatosis

• Endocrine: 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, uremia

• Lymphatic abnormalities: Chylothorax, malignancy, lymphangiectasia

• Malignancy: Carcinoma, lymphoma, leukemia, mesothelioma, paraproteinemia

Transudative effusions

Causes of transudative effusions include the following:

• Atelectasis: Due to increased negative intrapleural pressure

• Cerebrospinal fluid (CSF) leak into pleural space: Thoracic spine injury, ventriculoperitoneal (VP) shunt dysfunction

• Heart failure

• Hepatic hydrothorax

• Hypoalbuminemia

• Iatrogenic: Misplaced catheter into lung

• Nephrotic syndrome

• Peritoneal dialysis

• Urinothorax: Due to obstructive uropathy

Exceptions

The following are processes that typically cause exudative effusions, but may cause transudative effusions:

• Amyloidosis

• Chylothorax

• Constrictive pericarditis

• Hypothyroid pleural effusion

• Malignancy

• Pulmonary embolism

• Sarcoidosis

• Superior vena cava obstruction

• Trapped lung

Leading causes of pleural effusion

Leading causes of pleural effusion include the following:

• Congestive heart failure (transudate), incidence 500,000/year

• Pneumonia (exudate), incidence 300,000/year

• Cancer (exudate), incidence 200,000/year

• Pulmonary embolus (transudate or exudate), incidence 150,000/year

• Viral disease (exudate), incidence 100,000/year

• Coronary-artery bypass surgery (exudate), incidence 60,000/year

• Cirrhosis with ascites (transudate), incidence 50,000/year

Test sensitivity and specificity for exudate

Table 1. Test Sensitivity and Specificity for Exudate (Open Table in a new window)