Reference — Labs & Diagnostics
Pleural Fluid: Light's Criteria & Analysis Reference
When a thoracentesis returns fluid, the first question is whether the effusion is a transudate or an exudate. Light's criteria answer that, and the additional fluid studies — pH, glucose, LDH, cell counts, ADA, amylase, and triglycerides — narrow the cause and flag effusions that need drainage.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Pleural fluid analysis begins with one branch point: is the effusion a transudate or an exudate? A transudate forms when systemic forces (hydrostatic or oncotic pressure) push fluid across intact pleural capillaries — the pleura itself is healthy, and the fix is to treat the underlying condition such as heart failure. An exudate forms when local pleural or lung disease makes the capillaries leaky, so the fluid is protein- and LDH-rich and demands a search for the local cause. Light’s criteria separate the two, and the remaining chemistry, cell counts, and special studies pin down what is driving the effusion and whether it needs to be drained.
Light's criteria
The effusion is an exudate if any one of the following is met:
- Pleural fluid / serum protein ratio > 0.5
- Pleural fluid / serum LDH ratio > 0.6
- Pleural fluid LDH > two-thirds the upper limit of normal for serum LDH
If none of the three are met, the effusion is a transudate. Because the rule classifies as an exudate when any single criterion is positive, it is highly sensitive for exudates — at the cost of occasionally over-calling them.
Causes by category
Once an effusion is classified, the differential narrows. Transudates trace back to a systemic problem; exudates point to disease at the pleura or in the lung.
| Category | Mechanism | Common causes |
|---|---|---|
| Transudate | Systemic imbalance across intact pleural capillaries | Congestive heart failure (most common), cirrhosis / hepatic hydrothorax, nephrotic syndrome, hypoalbuminemia |
| Exudate | Capillary leak from local pleural or lung disease | Parapneumonic effusion / empyema, malignancy, pulmonary embolism, tuberculosis, pancreatitis, connective tissue disease |
Additional fluid studies
Beyond Light’s criteria, targeted studies refine the diagnosis and identify effusions that need a chest tube rather than antibiotics alone.
| Test | Finding | Significance |
|---|---|---|
| Pleural fluid pH | < 7.20 | Complicated parapneumonic effusion or empyema — needs drainage. Collect anaerobically on ice and run promptly, like a blood gas. |
| Glucose | < 60 mg/dL | Complicated parapneumonic effusion, empyema; also rheumatoid and malignant effusions. |
| LDH | High | Marks an exudate; very high values point toward empyema. |
| Cell count / differential | Neutrophil-predominant | Acute parapneumonic effusion. |
| Cell count / differential | Lymphocyte-predominant | Tuberculosis or malignancy. |
| Appearance | Grossly bloody | Malignancy, pulmonary embolism, or trauma. |
| Appearance | Frank pus / positive Gram stain | Empyema — drain. |
| Adenosine deaminase (ADA) | Elevated | Tuberculous effusion. |
| Amylase | Elevated | Pancreatitis or esophageal rupture. |
| Triglycerides | > 110 mg/dL | Chylothorax. |
Clinical Notes
- Light’s criteria over-call exudates in diuresed heart failure.The rule is highly sensitive for exudates but can misclassify a transudate as an exudate after diuresis concentrates the fluid’s protein and LDH. When the clinical picture says heart failure but the criteria say exudate, reconcile the two before chasing a local cause.
- Complicated-effusion chemistry triggers drainage. A pleural pH < 7.20, glucose < 60 mg/dL, or frank pus marks a complicated parapneumonic effusion or empyema that needs chest-tube drainage rather than antibiotics alone.
- Handle the pH specimen like a blood gas.Collect it anaerobically, keep it on ice, and run it promptly — air exposure and delay falsely shift the pH and can change the drainage decision.
- Reference cutoffs are method- and lab-specific. The thresholds here reflect widely used values; confirm the upper limit of normal for serum LDH and the glucose and triglyceride cutoffs against your own laboratory’s reference ranges.
Related Resources
Sources
- 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.
- Hooper C, Lee YCG, Maskell N; BTS Pleural Guideline Group. Investigation of a unilateral pleural effusion in adults: British Thoracic Society Pleural Disease Guideline 2010. Thorax. 2010;65(Suppl 2):ii4-ii17.
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Pleural disease.