Pleural Effusion: MCCQE1 Study Guide

Introduction

Pleural effusion is a common clinical presentation encountered in both inpatient and outpatient settings across Canada. For the MCCQE1, understanding the pathophysiology, diagnostic algorithm (specifically Light’s Criteria), and management of pleural effusions is critical. This topic falls under the Medical Expert role of the CanMEDS framework, requiring candidates to demonstrate diagnostic reasoning and management skills.

A pleural effusion is defined as an abnormal accumulation of fluid in the pleural space. In healthy individuals, approximately 10–20 mL of fluid exists in the pleural space to facilitate lung movement.

Pathophysiology and Classification

The fundamental step in analyzing a pleural effusion is differentiating between a Transudate and an Exudate. This distinction guides the subsequent workup and management.

Light’s Criteria

This is the gold standard for differentiating transudative from exudative effusions.

Transudate vs. Exudate: Etiology

Clinical Presentation

History

• Dyspnea: The most common symptom.
• Pleuritic Chest Pain: Sharp pain worsened by inspiration (suggests inflammation).
• Cough: Usually dry/non-productive.
• Constitutional Symptoms: Fever (infection), weight loss (malignancy/TB), night sweats (TB/malignancy).

Physical Examination

The physical exam findings are high-yield for the MCCQE1.

Diagnostic Approach

Interpreting Pleural Fluid Results

Management

Management depends entirely on the underlying etiology.

1. Parapneumonic Effusions & Empyema

This is a critical area for MCCQE1 decision-making.

2. Malignant Effusions

• Common in lung and breast cancer.
• Management:
  • Therapeutic thoracentesis for symptom relief.
  • Recurrent: Indwelling Pleural Catheter (IPC) (preferred in Canada for outpatient management) or Pleurodesis (talc).

3. Transudative Effusions (e.g., CHF)

• Treat the underlying cause (Diuretics).
• Thoracentesis only if refractory or atypical features present.

Canadian Guidelines & Clinical Pearls

• TB in Canada: While overall rates are low, always consider TB in patients from endemic regions or Indigenous communities, especially if the effusion is lymphocyte-predominant with a high protein count.
• Ultrasound Guidance: Health Canada and provincial bodies strongly advocate for ultrasound-guided thoracentesis to reduce the risk of iatrogenic pneumothorax.

Mnemonic: “LIGHT” for Light’s Criteria

• LDH pleural/serum > 0.6
• Inflammation (Exudate)
• Greater than 0.5 (Protein ratio)
• High Protein
• Two-thirds (Pleural LDH > 2/3 upper limit serum LDH)

Key Points to Remember for MCCQE1

• Light’s Criteria is the definitive method to distinguish transudate from exudate.
• CHF is the most common cause of transudative effusion; Pneumonia is the most common cause of exudative.
• A pleural fluid pH < 7.20 in a parapneumonic effusion is an indication for drainage (chest tube), not just antibiotics.
• Stony dullness on percussion is the hallmark physical sign.
• Always perform a diagnostic thoracentesis for a new unexplained effusion, except in clear-cut CHF.
• Rheumatoid Arthritis effusions typically have very low glucose (< 1.6 mmol/L) and high LDH.

Sample Question

Case Presentation

A 62-year-old male presents to the Emergency Department with a 5-day history of fever, productive cough, and progressive right-sided pleuritic chest pain. He has a past medical history of hypertension and type 2 diabetes.

On physical examination:

• Temperature: 38.9°C
• Heart Rate: 102 bpm
• Respiratory Rate: 24/min
• BP: 130/80 mmHg
• Oxygen Saturation: 93% on room air

Respiratory exam reveals decreased breath sounds and dullness to percussion over the right lower lung base. A chest X-ray confirms a right lower lobe consolidation with a moderate-sized pleural effusion.

A diagnostic thoracentesis is performed under ultrasound guidance. Fluid analysis results are as follows:

• Appearance: Turbid
• pH: 7.15
• Glucose: 2.8 mmol/L
• LDH: 1200 U/L
• Gram stain: Negative
• Protein: 45 g/L (Serum Protein: 70 g/L)

Which one of the following is the most appropriate next step in management?

Options

• A. Continue intravenous antibiotics alone and repeat thoracentesis in 24 hours
• B. Insert a small-bore chest tube (thoracostomy)
• C. Perform flexible bronchoscopy
• D. Request a thoracic surgery consult for immediate decortication
• E. Administer intra-pleural fibrinolytics immediately

Explanation

The correct answer is:

• B. Insert a small-bore chest tube (thoracostomy)

Detailed Explanation:

This patient presents with a complicated parapneumonic effusion.

1. Diagnosis: The patient has pneumonia (fever, cough, consolidation) and a pleural effusion. The fluid analysis confirms an exudate (Pleural Protein/Serum Protein = 45/70 = 0.64, which is > 0.5).
2. Risk Stratification: The key to answering this question is interpreting the pleural fluid markers to decide between simple and complicated effusions.
   • Simple Parapneumonic Effusion: pH > 7.20, Glucose > 3.3 mmol/L, LDH < 1000 IU/L. Managed with antibiotics alone.
   • Complicated Parapneumonic Effusion: pH < 7.20, Glucose < 3.3 mmol/L, or LDH > 1000 IU/L, or positive Gram stain/culture (without frank pus).
3. Application: This patient has a pH of 7.15 (low), Glucose of 2.8 (low), and LDH of 1200 (high). This defines a complicated parapneumonic effusion.
4. Management: Complicated effusions require drainage (source control) in addition to antibiotics to prevent progression to empyema and fibrosis (trapped lung). A small-bore chest tube is the standard initial intervention for drainage.

Why other options are incorrect:

• A: Antibiotics alone are insufficient for complicated effusions (pH < 7.20) and may lead to treatment failure.
• C: Bronchoscopy is indicated for suspected endobronchial obstruction (e.g., cancer) or to obtain culture if sputum is negative, but drainage of the pleural space is the priority here.
• D: Surgery (VATS/decortication) is usually reserved for patients who fail tube thoracostomy or have complex multiloculated empyema. It is not the first-line step before attempting drainage.
• E: Fibrinolytics (e.g., tPA/DNase) are used for loculated effusions that do not drain well with a chest tube alone. They are not the immediate first step before a tube is even inserted.

References

1. Toronto Notes 2024. Respirology Chapter: Pleural Effusion. Toronto Notes for Medical Students, Inc.
2. Light RW. Pleural Effusion. N Engl J Med 2002; 346:1971-1977.
3. British Thoracic Society. BTS Pleural Disease Guideline 2010 (and updates). Link 
4. Choosing Wisely Canada. Respiratory Medicine. Link 
5. McMaster Textbook of Internal Medicine. Pleural Effusion. Canadian Edition.