How should a pleural effusion be assessed and evaluated?

Assessment of Pleural Effusion

Begin with a thorough clinical history and physical examination to determine if the effusion is likely a transudate (which often does not require invasive sampling) or an exudate (which requires diagnostic thoracentesis), followed by thoracic ultrasound and diagnostic aspiration when indicated. 1

Initial Clinical Evaluation

History and Physical Examination

• Obtain a detailed drug history as tyrosine kinase inhibitors and other medications commonly cause exudative pleural effusions; use the Pneumotox app for comprehensive drug-related pleural disease information 1
• Document occupational history, particularly any asbestos exposure, which is vital when investigating all pleural effusions 1
• Assess for pulmonary embolism: approximately 75% of patients with pulmonary embolism and pleural effusion have pleuritic pain, with effusions typically occupying less than one-third of the hemithorax and dyspnea disproportionate to effusion size 1
• Identify transudate features clinically: in settings strongly suggestive of heart failure, cirrhosis, hypoalbuminemia, or dialysis with bilateral effusions and typical features, aspiration is not required unless atypical features exist or the effusion fails to respond to treatment 1, 2

Imaging Strategy

• Perform chest radiography as the initial imaging modality to determine laterality and detect moderate to large effusions 1, 3
• Conduct thoracic ultrasound (TUS) on every patient at initial presentation before any pleural procedure, as it is now an extension of the physician's examination 1
• Use TUS to answer: "Is it safe to perform diagnostic aspiration?" and to assess effusion size, character, and signs of malignancy (diaphragmatic and parietal pleural nodularity) 1
• Order contrast-enhanced CT chest (venous phase for pleural enhancement) if aspiration is unsafe, if malignancy is suspected (include chest, abdomen, and pelvis), or if the diagnosis remains unclear after initial evaluation 1, 2

Diagnostic Thoracentesis Protocol

Indications for Aspiration

Do not aspirate bilateral effusions in clinical settings strongly suggestive of transudate unless atypical features are present, normal heart size is seen on chest radiograph, effusions are unilateral or asymmetric, there is progressive enlargement, or no response to treatment occurs 1, 4

Perform diagnostic thoracentesis for:

• Unilateral effusions 1
• Exudative effusions by clinical assessment 2
• Suspected parapneumonic effusion or empyema 1, 2
• Any effusion where the diagnosis is uncertain 1, 2

Aspiration Technique

• Use a fine-bore 21-gauge needle with a 50 mL syringe for diagnostic sampling 1, 2
• Perform ultrasound guidance for all thoracentesis procedures, as it achieves 97% success rate even for small or loculated effusions and reduces complications 4, 3
• Do not attempt thoracentesis if effusion measures <1 cm thickness on lateral decubitus view due to increased complication risk 4
• Limit fluid removal to ≤1.5 liters in a single thoracentesis to avoid re-expansion pulmonary edema 4

Pleural Fluid Analysis

Send samples in both sterile vials and blood culture bottles to increase diagnostic yield 1, 2

Routine analysis includes: 1, 2

• Appearance and odor of fluid
• Protein and lactate dehydrogenase (LDH) levels
• pH (in all non-purulent effusions when infection is suspected)
• Gram stain and aerobic/anaerobic cultures
• Acid-fast bacilli (AAFB) stain and tuberculosis culture
• Cytology
• Cell count with differential

Apply Light's criteria when pleural protein is 25-35 g/L to differentiate transudate from exudate: 1, 2

• Pleural protein <25 g/L = transudate
• Pleural protein >35 g/L = exudate
• Pleural protein 25-35 g/L = use Light's criteria

Obtain anaerobic cultures when aspiration pneumonia is suspected 5

Interpretation and Next Steps

For Transudates

• Treat the underlying cause (heart failure, cirrhosis, nephrotic syndrome) 2
• Reassess if no response to treatment or if atypical features develop 1

For Exudates Without Clear Diagnosis

Order contrast-enhanced chest CT with fluid present for better pleural visualization and to identify optimal biopsy sites 1, 2

Obtain pleural tissue if cytology is non-diagnostic (only 60% of malignant effusions are diagnosed by cytology alone) through: 1, 2

• Ultrasound or CT-guided pleural biopsy
• Closed pleural biopsy
• Thoracoscopy

Reconsider diagnoses with specific treatments including tuberculosis, pulmonary embolism, lymphoma, IgG4 disease, and chronic heart failure 1

Consider watchful waiting with interval CT scans for persistently undiagnosed effusions in appropriate clinical contexts 1

For Parapneumonic Effusions

• pH <7.2 indicates complicated parapneumonic effusion requiring prompt consultation for catheter or chest tube drainage, possible tissue plasminogen activator/deoxyribonuclease therapy, or thoracoscopy 3
• Simple parapneumonic effusions (pH >7.2) usually resolve with antibiotics alone 5

Critical Pitfalls to Avoid

• Do not reflexively tap all bilateral effusions when clinical context clearly indicates transudate, as this leads to unnecessary procedures 4
• Do not assume bilateral effusions are always benign, as malignancy can cause bilateral effusions in some cases 4
• Do not delay thoracentesis in patients with fever and effusion, as parapneumonic effusions require early sampling to prevent progression to empyema 4
• Do not miss pulmonary embolism as pleural fluid tests are unhelpful in diagnosis; maintain high clinical suspicion 1
• Do not perform thoracentesis without ultrasound guidance in the modern era, as it significantly reduces complications 1, 3

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