Light's Criteria
Differentiation between Exudate and Transudate
Result
Calculated values:
About Light's Criteria
A pleural effusion is classified as an EXUDATE if at least one of the following criteria is met:
- Pleural fluid protein / serum protein ratio > 0.5
- Pleural fluid LDH / serum LDH ratio > 0.6
- Pleural fluid LDH > 2/3 of the upper limit of normal for serum LDH
The criteria have high sensitivity for identifying exudates (approximately 98%), with specificity of around 75–83% depending on the population studied (Heffner et al., Chest 1997). The reduced specificity means some transudates may be incorrectly classified as exudates.
In the UK, the British Thoracic Society (BTS) endorses Light's Criteria as the first-line method for differentiating exudates from transudates in the investigation of a unilateral pleural effusion.
Common causes of Exudate
- • Pneumonia / parapneumonic effusion
- • Tuberculosis
- • Malignancy
- • Pulmonary embolism *
- • Connective tissue diseases
- • Pancreatitis
* Pleural effusion secondary to PE may be classified as either exudate or transudate by Light's Criteria
Common causes of Transudate
- • Heart failure *
- • Liver cirrhosis
- • Nephrotic syndrome
- • Peritoneal dialysis
- • Hypoalbuminaemia
- • Atelectasis
* Diuretic use may lead to misclassification as exudate — see clinical note below
Clinical note: In patients with heart failure receiving diuretics, Light's Criteria may incorrectly classify a transudate as an exudate due to concentration of pleural proteins. In such cases, the serum-to-pleural albumin gradient (serum albumin − pleural albumin > 1.2 g/dL suggests transudate) may assist in differentiation.
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. PubMed ↗
- 2. Heffner JE, Brown LK, Barbieri CA. Diagnostic value of tests that discriminate between exudative and transudative pleural effusions. Chest. 1997;111(4):970–980. PubMed ↗
- 3. Chauhan A, et al. Comparison of the Efficacy of Light's Criteria With Serum-Effusion Albumin Gradient and Pleural Effusion Glucose. Cureus. 2023;15(7):e42008. PubMed ↗