Pleural Effusion
Fluid in the pleural space. Step 1: transudative or exudative (Light's criteria). Step 2: if exudative, find the cause. Step 3: drain if needed. You will do thoracenteses -know Light's cold.
🔍 Overview
Light's Criteria -Exudate if ANY ONE Met
If any ONE of these three is positive → exudate. All three negative → transudate.
| Criterion | Exudate Cutoff |
|---|---|
| Pleural protein / Serum protein | > 0.5 |
| Pleural LDH / Serum LDH | > 0.6 |
| Pleural LDH | > 2/3 upper limit of normal for serum LDH |
Light's criteria misclassify ~25% of transudates as exudates (especially in HF patients on diuretics). If you suspect a transudative effusion misclassified as exudate → check serum-pleural albumin gradient. If > 1.2 g/dL → transudate despite Light's.
Common Causes
| Transudative | Exudative |
|---|---|
| CHF (most common overall) | Pneumonia / parapneumonic (most common exudate) |
| Hepatic hydrothorax (cirrhosis) | Malignancy (lung, breast, lymphoma) |
| Nephrotic syndrome | PE |
| Peritoneal dialysis | TB (lymphocyte-predominant, ADA > 40) |
| Hypothyroidism | Autoimmune (SLE, RA) |
| Pancreatitis (elevated amylase) | |
| Esophageal rupture (low pH, high amylase) |
Pleural Fluid Analysis -What to Send
| Always Send | If Indicated |
|---|---|
| Cell count with differential | Cytology (if malignancy suspected -send ≥ 60 mL) |
| Protein, LDH, glucose | ADA (adenosine deaminase) -TB (> 40 suggestive) |
| Gram stain, culture | Amylase -pancreatitis, esophageal rupture |
| pH | Triglycerides -chylothorax (> 110 mg/dL) |
| + serum protein, LDH, albumin (same day) | Hematocrit -hemothorax (pleural Hct > 50% of blood) |
🚨 Management
When to Tap
- New effusion of unknown etiology -diagnostic thoracentesis
- Clinically significant (dyspnea) -therapeutic thoracentesis (remove up to 1.5 L per session)
- Suspected empyema or complicated parapneumonic -emergent drainage
Do NOT tap if: bilateral symmetric effusions in a patient with clear CHF (treat the HF first -they'll resolve). Only tap if unilateral, asymmetric, febrile, or not responding to diuresis.
Complicated Parapneumonic / Empyema
| Category | Fluid Features | Management |
|---|---|---|
| Simple parapneumonic | Clear, pH > 7.2, glucose > 60, LDH < 1000, culture negative | Antibiotics alone. May not need drainage. |
| Complicated parapneumonic | pH < 7.2, glucose < 60, LDH > 1000, or positive gram stain/culture | Chest tube drainage + antibiotics. Consider tPA/DNase instillation MIST2, 2011. |
| Empyema | Frankly purulent fluid or positive culture | Chest tube drainage mandatory. If loculated or not draining → tPA/DNase or VATS. Prolonged antibiotics (3–6 weeks). |
Malignant Effusion
- Positive cytology or biopsy confirms malignancy
- Recurrent → indwelling pleural catheter (IPC) or talc pleurodesis
- Median survival with malignant effusion: 3–12 months (depends on primary cancer)
- IPC allows outpatient drainage, avoids repeated thoracenteses. Can achieve spontaneous pleurodesis in ~50%.
📋 On Rounds
Light's criteria classify a CHF patient's effusion as exudative. What do you do?
Light's criteria misclassify ~25% of transudates as exudates, especially in CHF patients on diuretics (diuresis concentrates pleural protein and LDH, pushing ratios above exudative cutoffs). Check the serum-pleural albumin gradient: if > 1.2 g/dL → the effusion is truly transudative despite meeting Light's criteria.
What pleural fluid pH tells you and when it changes management?
pH < 7.2 in a parapneumonic effusion = complicated → needs chest tube. Low pH means bacteria are metabolizing glucose → producing CO₂ and lactic acid → acidifying the fluid. This also correlates with high LDH and low glucose. In empyema, pH can drop below 7.0. Important: collect pH in a heparinized blood gas syringe, not a regular tube. Also: low pH in a non-infected effusion → think malignancy, esophageal rupture, or rheumatoid pleurisy.
A pleural fluid has protein ratio 0.32, LDH ratio 0.58, and LDH 180 (ULN = 200). Transudate or exudate?
Exudate. Light's criteria require meeting any 1 of 3: (1) Fluid/serum protein ratio > 0.5 (this is 0.32 -NOT met), (2) Fluid/serum LDH ratio > 0.6 (this is 0.58 -NOT met), (3) Fluid LDH > 2/3 of serum ULN (2/3 × 200 = 133; fluid LDH is 180 -MET). One criterion positive = exudate. Common trap: Light's criteria misclassify ~25% of transudates as exudates (especially in diuresed CHF patients -diuretics concentrate the fluid).
How do you manage a complicated parapneumonic effusion vs empyema?
Uncomplicated parapneumonic: fluid is free-flowing, pH > 7.2, glucose > 60, LDH < 1000, gram stain/culture negative. Treatment: antibiotics alone, no chest tube. Complicated parapneumonic: pH < 7.2, glucose < 60, LDH > 1000, or positive gram stain/culture -but no frank pus. Loculations may be present. Treatment: chest tube + antibiotics.
Clinical Examples
📋 Case 1, CHF Effusion Misclassified by Light's Criteria
Patient: 74M, HFrEF (EF 25%), on aggressive diuresis with IV furosemide. Large right-sided pleural effusion tapped for dyspnea relief.
Key findings: Pleural fluid: protein ratio 0.55, LDH ratio 0.52, LDH 160 (ULN 200). Meets 1 of 3 Light's criteria (protein ratio > 0.5) → classified as exudate. But: bilateral LE edema, JVD, BNP 2,800.
Management:
- Check serum-pleural albumin gradient: 1.8 g/dL (> 1.2 confirms true transudate)
- This is a Light's criteria false-positive, diuretics concentrated the pleural protein
- No further invasive workup needed; treat underlying CHF
- Optimize diuresis (uptitrate furosemide, add metolazone if needed)
- If recurrent despite optimal HF management → consider tunneled pleural catheter
Teaching point: Light's criteria misclassify ~25% of CHF transudates as exudates, especially in diuresed patients. The serum-effusion albumin gradient (> 1.2 g/dL = transudate) corrects for this. Always apply clinical context before ordering extensive exudative workup.
📋 Case 2, Complicated Parapneumonic Effusion
Patient: 56F, DM2 and alcohol use disorder, admitted with RLL pneumonia 5 days ago on ceftriaxone + azithromycin. Persistent fever despite antibiotics, worsening dyspnea.
Key findings: CXR: enlarging right-sided effusion with loculations on ultrasound. Thoracentesis: turbid fluid, pH 6.9, glucose 28, LDH 2,800, gram stain: gram-positive cocci in chains. Protein ratio 0.8.
Management:
- Chest tube placement, complicated parapneumonic effusion (pH < 7.2, glucose < 60, positive gram stain)
- Broaden antibiotics: vancomycin + piperacillin-tazobactam
- Intrapleural tPA (10 mg) + DNase (5 mg) BID x 3 days for loculated effusion
- CT surgery consult for VATS if inadequate drainage after tPA/DNase
- Repeat imaging in 24-48h to assess drainage adequacy
Teaching point: Pleural fluid pH is the single most important test for determining if a parapneumonic effusion needs drainage. pH < 7.2 = complicated = chest tube. MIST2, 2011 showed that combination tPA + DNase (not either alone) significantly improved fluid drainage and reduced surgical referral.
📋 Case 3, Malignant Pleural Effusion
Patient: 68F, never-smoker, presents with 3 months of progressive dyspnea and 15-lb weight loss. No fever, no cough. CXR: massive left-sided effusion with contralateral mediastinal shift.
Key findings: Thoracentesis: 2L bloody fluid. Exudate by Light's criteria (protein ratio 0.72, LDH ratio 0.85). Cytology: adenocarcinoma (TTF-1 positive, consistent with lung primary). Glucose 42, pH 7.18.
Management:
- Symptomatic improvement with large-volume thoracentesis (limit to 1.5L per session to avoid re-expansion pulmonary edema)
- Low pH + low glucose in malignant effusion = high tumor burden, poor prognosis
- For recurrent effusion: tunneled pleural catheter (PleurX) preferred over pleurodesis for most patients
- Oncology referral for staging and driver mutation testing (EGFR, ALK, ROS1)
- Goals of care discussion given advanced malignancy
Teaching point: Malignant effusions are exudative and often bloody. Low glucose and low pH in a malignant effusion predict poor survival and failed pleurodesis. Tunneled pleural catheters allow outpatient drainage and achieve spontaneous pleurodesis in ~45% of patients. Lung and breast cancer are the most common causes.
📣 Sample Presentation
One-Liner
"Mr. Hernandez is a 68-year-old with CHF presenting with worsening dyspnea. CXR shows large left-sided pleural effusion. Thoracentesis: protein ratio 0.28, LDH ratio 0.32. Transudative by Light's criteria."
Key Points to Cover on Rounds
Large left pleural effusion -transudative (protein ratio 0.28, LDH ratio 0.32, LDH 108 -all below Light's cutoffs). Etiology: CHF (bilateral LE edema, elevated BNP 1,800, known HFrEF). 1.5L drained with symptomatic improvement. Fluid: clear, straw-colored. Cell count and cytology sent (expected benign). No further invasive workup needed for transudative effusion with clear etiology. Treatment: diuresis optimization (furosemide uptitrated). Plan: if recurrent despite optimal diuresis → consider tunneled pleural catheter. Not empyema, not malignant based on presentation.
🧪 Workup
Workup
See the Overview and Management tabs for the pleural effusion workup algorithm (CXR → thoracic ultrasound → diagnostic thoracentesis with Light's criteria, pleural fluid studies, and pleural biopsy when malignancy or TB is suspected).
💊 Medications
Medications
Medication details (antibiotics for parapneumonic/empyema, intrapleural tPA + DNase for loculated empyema per MIST-2, pleurodesis agents for malignant effusion) are in the Management tab with evidence-based dosing and trial citations.
⚡ Summary
Summary
Light's Criteria: Any One Means Exudate
Pleural-to-serum protein ratio above 0.5, pleural-to-serum LDH ratio above 0.6, or pleural LDH above two-thirds the upper limit of normal serum LDH. All three negative means transudate. The criteria are highly sensitive for exudates and therefore misclassify some transudates as exudates, particularly after diuresis.
Check the Albumin Gradient When Diuretics Are On Board
A diuresed heart failure patient concentrates pleural protein and can be misclassified as exudative. A serum-to-pleural albumin gradient above 1.2 g/dL indicates a transudate regardless of Light's criteria, and it rescues these patients from an unnecessary exudate workup.
The Two Categories Have Different Differentials
Transudate: heart failure (by far the commonest), cirrhosis, nephrotic syndrome, and it means the pleura itself is normal and the problem is hydrostatic or oncotic. Exudate: parapneumonic effusion, malignancy, pulmonary embolism, tuberculosis, and connective tissue disease, meaning the pleura is inflamed or leaking.
Send the Right Studies
Protein, LDH, pH, glucose, cell count with differential, Gram stain and culture, and cytology if malignancy is possible. Add adenosine deaminase for suspected tuberculosis, triglycerides for chylothorax, and amylase for pancreatic or esophageal causes. pH must be collected anaerobically in a blood gas syringe and run promptly, or it is meaningless.
pH Decides Whether the Tube Goes In
A parapneumonic effusion with pH below 7.20, glucose below 60 mg/dL, a positive Gram stain or culture, or frank pus is a complicated effusion or empyema and needs chest tube drainage. Antibiotics alone do not clear an infected pleural space, and a delayed tube leads to loculation and a decortication.
Break Loculations Rather Than Escalating Antibiotics
Intrapleural tPA plus DNase improves drainage and reduces the need for surgery in loculated pleural infection (MIST2). Neither agent alone works. Refer for VATS decortication when drainage fails, and do not simply keep the tube in place on suction hoping it will resolve.
Malignant Effusions Recur
Cytology is positive in only about 60% on a first tap, so a negative result does not exclude malignancy: repeat it or proceed to pleural biopsy. Because they recur, definitive management is pleurodesis or an indwelling pleural catheter rather than repeated therapeutic taps.
Drain Slowly
Remove no more than about 1.5 liters at one sitting, and stop for chest pain, cough or a fall in pleural pressure. Rapid or large-volume drainage causes re-expansion pulmonary edema, particularly when the lung has been collapsed for a long time. Use ultrasound guidance, which reduces pneumothorax and dry taps substantially.
📄 One Pager
Text version
Pulmonology · One Pager
Pleural Effusion
Thoracentesis → Light's criteria → transudate (treat cause) or exudate (further workup). Parapneumonic: pH/glucose/LDH determine if chest tube needed.
Light's Criteria
Exudate if ANY 1: protein ratio > 0.5, LDH ratio > 0.6, fluid LDH > 2/3 serum ULN. Sensitive but 25% false-positive for transudates (diuresed CHF). Check albumin gradient if discordant.
Workup by Type
Transudate: CHF, cirrhosis, nephrotic → treat underlying cause. Exudate: send cell count, gram stain, culture, glucose, pH, LDH, cytology. Lymphocytic → TB (ADA), malignancy.
Parapneumonic Management
Uncomplicated (pH > 7.2, glc > 60): abx only. Complicated (pH < 7.2, glc < 60, +culture): chest tube + abx. Empyema (frank pus): chest tube + surgery consult. tPA/DNase for loculated effusions MIST2, 2011.
Key Drugs
Diagnostic thoracentesisAll new effusions
Albumin gradientIf Light's doesn't match clinical picture
tPA + DNaseLoculated parapneumonic [MIST2
PleurX catheterRecurrent malignant effusion
Pitfalls
- Not doing thoracentesis on new effusion (it's the test)
- Missing false exudate in diuresed CHF (check albumin gradient)
- Not sending pH/glucose on parapneumonic effusion (determines tube need)
- Chest tube delay in empyema
Related Topics
AsthmaCOPD -Chronic ManagementHemoptysisInterstitial Lung DiseaseObstructive Sleep ApneaPFT Interpretation