Light's Criteria
Differentiates between exudative and transudative pleural effusions.
Light's Criteria: Exudate or Transudate?
Light's Criteria, developed in 1972 by Dr. Richard W. Light, remains the gold standard in pulmonology and internal medicine for pleural fluid analysis. Their primary role is to differentiate a pleural effusion caused by mechanical factors (Transudate) from one related to inflammation, infection, or malignancy (Exudate).
How to interpret the results?
An effusion is classified as an EXUDATE if at least ONE of the following three criteria is met. If none are met, it is classified as a TRANSUDATE.
- 1. Protein Ratio: Pleural Fluid Protein / Serum Protein > 0.5
- 2. LDH Ratio: Pleural Fluid LDH / Serum LDH > 0.6
- 3. Absolute Pleural LDH: Greater than two-thirds (2/3) of the laboratory's upper limit of normal (ULN) for serum LDH.
Diagnostic Implications
Transudate: Usually bilateral. Common causes: Heart failure (most common), hepatic cirrhosis, nephrotic syndrome. Treatment targets the underlying cause (e.g., diuretics).
Exudate: Often unilateral. Requires further investigation (cytology, bacteriology, biopsies). Common causes: Pneumonia (parapneumonic effusion), tuberculosis, cancer, pulmonary embolism.
Summary of Differential Diagnoses
| Fluid Type | Pathophysiology | Common Etiologies |
|---|---|---|
| Transudate | Pressure imbalance (↑ hydrostatic or ↓ oncotic). Healthy pleura. | Heart failure, Cirrhosis, Nephrotic syndrome. |
| Exudate | Altered local capillary permeability. Pleural inflammation. | Infection (Pneumonia, TB), Malignancy, Pulmonary Embolism. |
References:
1. Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507-513.
[Ann Intern Med]
2. Porcel JM. Identifying transudates misclassified by Light's criteria. Curr Opin Pulm Med. 2013;19(4):362-367.
[LWW Journals]
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