What it is: A standard set of three biochemical rules to categorise pleural effusions as transudative or exudative. If any one of the three criteria is met, the fluid is classified as an exudate.
Why it matters: It directs the entire diagnostic pathway. Transudates suggest a systemic issue (organ failure, volume overload) requiring diuresis and medical optimisation. Exudates imply local pleural disease (infection, malignancy, inflammation) demanding further local investigations like cytology, cultures, or pleural biopsy.
What it is: Approximately 20-30% of transudates in heart failure patients who are actively receiving diuretics will be falsely classified as exudates by Light's criteria.
The Bedside Pearl: Diuresis concentrates protein and LDH in the pleural space faster than they can equilibrate with the serum. If the clinical picture screams heart failure but Light's criteria says exudate, calculate the Serum-Pleural Albumin Gradient (SPAG). A SPAG > 1.2 g/dL proves the fluid is actually a transudate.
Local Epidemiology: In India, tuberculosis remains the most common aetiology of exudative pleural effusions, especially in young patients. Malignancy follows closely, typically in older demographics.
The Bedside Pearl: Routine pleural fluid AFB smears have very low yield (< 5%). Instead, an Adenosine Deaminase (ADA) level > 40 U/L in a lymphocyte-predominant exudate is highly sensitive and specific for TB pleuritis. Always send fluid for GeneXpert/CBNAAT and mycobacterial culture to confirm and check for drug resistance.
| Transudates (Systemic) | Exudates (Local) |
|---|---|
| Heart failure (Most common) | Tuberculosis (High prevalence in India) |
| Hepatic cirrhosis (Hepatic hydrothorax) | Parapneumonic effusion / Empyema |
| Nephrotic syndrome | Malignancy (Lung, breast, lymphoma) |
| Severe hypoalbuminaemia / Malnutrition | Pulmonary Embolism (Can be either, usually exudate) |
| Peritoneal dialysis | Connective tissue disease (RA, SLE) |
AMA Style:
MEDiscuss. Light's Criteria & Pleural Fluid Analysis. MEDiscuss.org. Published 2026. Accessed .
