About Light's Criteria for pleural effusion
Light's Criteria, published by Richard Light and colleagues in 1972, are the standard bedside rule for separating an exudative pleural effusion from a transudative one. The test compares protein and lactate dehydrogenase (LDH) in the pleural fluid against the same measurements in serum, drawn at roughly the same time. An effusion is classified as exudative if it meets at least one of three ratio tests; if it meets none, it is classified as a transudate.
The three criteria
- Protein ratio: pleural fluid protein divided by serum protein is greater than 0.5.
- LDH ratio: pleural fluid LDH divided by serum LDH is greater than 0.6.
- Pleural LDH vs. lab ceiling: pleural fluid LDH is greater than two-thirds of the laboratory's upper limit of normal for serum LDH.
Because Light's Criteria only need one positive test to call an effusion exudative, the rule is intentionally sensitive — it rarely misses a true exudate — at the cost of occasionally flagging a transudate as exudate.
Putting the result in context
An exudative classification points toward a local pleural process such as infection (parapneumonic effusion or empyema), malignancy, or another inflammatory cause, and typically prompts further pleural fluid analysis (cell count, cytology, culture, glucose, pH). A transudative classification points toward a systemic process, most often heart failure, cirrhosis with ascites, or nephrotic syndrome, where the underlying condition rather than the pleura itself is treated. The best-known limitation is that diuretic use for heart failure can concentrate pleural protein and LDH enough to cross the exudate cutoffs even though the underlying process is a transudate; in that setting a serum minus pleural fluid albumin gradient above 1.2 g/dL supports the transudate diagnosis despite a positive Light's Criteria result.
When to consult a professional
This calculator performs the standard Light's Criteria arithmetic for education and reference. It does not replace clinical judgment, thoracentesis, or the additional pleural fluid studies a clinician orders once a fluid is characterized. Any decision about diagnosis or treatment of a pleural effusion should be made with a licensed healthcare provider who has the full clinical picture.