- Light's criteria are used to determine the nature of a pleural fluid sample, and narrow down the differential diagnosis of a pleural effusion.
- A pleural fluid sample is most likely exudate if one or more of the criteria are met.
- Pleural fluid : serum protein ratio >0.5
- Pleural fluid : serum LDH ratio >0.6
- Pleural fluid LDH > ⅔ of upper limit of normal serum LDH
Causes of Positive Light's Criteria
- Infection - parapneumonic effusion, empyema, viral, fungal, TB, ruptured abscess / cyst
- Pulmonary embolism (90% exudative)
- Autoimmune disease - rheumatoid pleuritis, SLE
- Oesophageal rupture