What Light's Criteria Measure
Light's criteria are used to classify pleural effusions as exudative or transudative using paired pleural fluid and serum biochemistry.
An effusion is classified as exudative if any one of the three criteria is positive. If none are positive, the effusion is classified as transudative.
Diagnostic Criteria
Criterion 1 uses the pleural fluid to serum protein ratio and is positive when the ratio is greater than 0.5.
Criterion 2 uses the pleural fluid to serum LDH ratio and is positive when the ratio is greater than 0.6.
Criterion 3 is positive when pleural fluid LDH exceeds two-thirds of the upper limit of normal for serum LDH.
Classification and Interpretation
Exudative Pleural Effusion:
At least one Light's criterion is positive. This supports an exudative process and usually warrants evaluation for local pleural or pulmonary disease.
Transudative Pleural Effusion:
None of the Light's criteria are positive. This supports a transudative process, often related to systemic hydrostatic or oncotic pressure imbalance.
Important Limitations
Light's criteria prioritize sensitivity for exudates and can overclassify some transudates as exudates in selected clinical settings.
- Diuretic-treated heart failure can produce pseudoexudates.
- Albumin or protein gradients may help in selected borderline cases.
- Light's criteria should be interpreted together with the overall clinical picture and pleural fluid analysis.