Guide — Labs & Diagnostics
Pleural Fluid Analysis & Light's Criteria
A thoracentesis result only helps if you can read it. This guide walks the first fork — transudate versus exudate by Light's criteria — and the pH, glucose, LDH, and cell counts that flag a complicated effusion needing a chest tube.
8 min read · Labs & Diagnostics
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
When a needle comes out of the chest with fluid in the syringe, the lab work that follows answers two questions that change the plan at the bedside. First: is this a transudate from a systemic problem, or an exudate from disease in the pleura or lung? Second, if the fluid is infected: is this a simple effusion that antibiotics will clear, or a complicated effusion or empyema that needs a chest tube? Pleural fluid analysis is how you answer both, and the respiratory therapist is often the one assisting the procedure and protecting the specimen that decides the call.
The values are not hard to read once you know which fork each one belongs to. Light’s criteria sort the transudate from the exudate, while the pH, glucose, and LDH — together with cell counts and a handful of special studies — sort the simple effusion from the one that needs drainage.
The first fork: transudate vs exudate
Every pleural effusion is first sorted into one of two buckets. A transudateis a systemic imbalance pushing protein-poor fluid across intact capillaries — the pleura itself is healthy. An exudate is local pleural or lung disease leaking protein-rich fluid into the space. This single split drives the entire differential, because the two buckets hold very different causes.
- Transudates: congestive heart failure (the most common cause overall), cirrhosis with hepatic hydrothorax, nephrotic syndrome, and hypoalbuminemia.
- Exudates: parapneumonic effusion and empyema, malignancy, pulmonary embolism, tuberculosis, pancreatitis, and connective tissue disease.
Light's criteria
Light’s criteria are the standard way to make the call. The fluid is an exudate if any one of the three thresholds below is met. If none are met, the fluid is a transudate.
| Test | Exudate threshold |
|---|---|
| Pleural fluid / serum protein ratio | Greater than 0.5 |
| Pleural fluid / serum LDH ratio | Greater than 0.6 |
| Pleural fluid LDH | Greater than two-thirds of the upper limit of normal for serum LDH |
The criteria are very sensitivefor exudates — they rarely miss one. The trade-off is that they occasionally misclassify a transudate as an exudate, especially in a heart-failure patient who has been diuresed, since diuresis concentrates the protein and LDH in the remaining fluid and nudges it across the threshold.
The complicated parapneumonic effusion & empyema
When a pneumonia seeds the pleural space, the fluid chemistry decides whether antibiotics alone will do or a chest tube is needed. The fluid marks a complicatedeffusion or empyema — and calls for drainage — when you see any of the following:
- Pleural fluid pH below 7.20.
- Glucose below 60 mg/dL.
- A high LDH.
- Frank pus, or a positive Gram stain or culture.
A practical detail that determines whether the pH is even usable: measure pleural pH on a blood gas analyzer, handled anaerobically and on ice, exactly like an arterial blood sample. A simple effusion with reassuring chemistry can be watched and treated medically; the complicated effusion and the empyema need the space drained.
Additional studies & special clues
Beyond Light’s criteria and the drainage chemistry, the cell count and a short list of special studies point toward a specific diagnosis.
| Study | What it suggests |
|---|---|
| Cell count & differential | Neutrophil-predominant in acute parapneumonic effusions; lymphocyte-predominant in tuberculosis and malignancy; grossly bloody in malignancy, PE, or trauma |
| Adenosine deaminase (ADA) | Elevated in tuberculous effusions |
| Amylase | Elevated with pancreatitis or esophageal rupture |
| Triglycerides | Above 110 mg/dL indicates a chylothorax |
| Cytology | Evaluates for malignancy |
What the RT does with it
The therapist’s role spans the procedure and the interpretation that follows.
- Protect the specimen. Assist with the thoracentesis and handle the pH sample anaerobically and on ice so the result stays valid.
- Correlate the data. Read the fluid result alongside the chest film and the clinical picture rather than in isolation.
- Recognize the drainage chemistry.Flag the complicated effusion or empyema — pH below 7.20, glucose below 60 mg/dL, high LDH, or pus — that calls for a chest tube.
- Anticipate what follows. Be ready for the drainage and the lung re-expansion that come after a complicated effusion is identified.
Common Pitfalls
- Mishandling the pH sample. Air exposure or a delay before analysis shifts the pH and invalidates one of the values that decides drainage.
- Forgetting the diuresed CHF transudate.Diuresis can push a true heart-failure transudate across Light’s thresholds, where it gets misread as an exudate.
- Treating an empyema with antibiotics alone. Complicated effusions and empyemas need the space drained, not just more antibiotics.
- Ignoring the special-study clues. Skipping the cell-count differential, ADA, amylase, triglycerides, or cytology can miss tuberculosis, chylothorax, or malignancy.
Board Exam Pearls
- Know the three Light’s criteriaand the “any one = exudate” rule cold.
- CHF is the top transudate and the most common cause of effusion overall.
- Pleural pH below 7.20 or glucose below 60 mg/dL means a complicated effusion needing drainage.
- Lymphocyte-predominant fluid suggests tuberculosis or malignancy.
- Handle pleural pH like a blood gas— anaerobic, on ice, run promptly.
FAQ
What are Light's criteria?
Light's criteria classify a pleural effusion as an exudate if it meets any one of three thresholds: a pleural fluid protein to serum protein ratio greater than 0.5, a pleural fluid LDH to serum LDH ratio greater than 0.6, or a pleural fluid LDH greater than two-thirds of the upper limit of normal for serum LDH. If none of the three are met, the fluid is a transudate. The rule is very sensitive, so it rarely misses a true exudate.
What separates a transudate from an exudate clinically?
A transudate comes from a systemic imbalance — congestive heart failure, cirrhosis, nephrotic syndrome, or hypoalbuminemia — pushing protein-poor fluid across intact capillaries. An exudate comes from local pleural or lung disease that leaks protein-rich fluid, such as a parapneumonic effusion, empyema, malignancy, pulmonary embolism, tuberculosis, pancreatitis, or connective tissue disease. The transudate-versus-exudate split drives the whole differential.
Which pleural fluid values mean a chest tube is needed?
In a parapneumonic effusion, a pleural fluid pH below 7.20, a glucose below 60 mg/dL, a high LDH, or frank pus or a positive Gram stain or culture marks a complicated effusion or empyema. That chemistry calls for drainage with a chest tube, not antibiotics alone.
Why is the pleural pH handled like a blood gas?
Pleural pH is measured on a blood gas analyzer, so the specimen must be collected anaerobically and kept on ice, exactly like an arterial blood gas sample. Air exposure or a delay before analysis shifts the pH and invalidates the result — and pH is one of the values that decides whether the effusion needs drainage.
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; thoracentesis.