What is the recommended evaluation and management approach for a pleural effusion in an adult presenting with new‑onset shortness of breath, chest discomfort, or decreased breath sounds?

Pleural Effusion: Evaluation and Management

Initial Clinical Assessment

Begin by determining whether the clinical picture suggests a transudate—if heart failure, cirrhosis, hypoalbuminemia, or dialysis is present with bilateral symmetric effusions, treat the underlying condition without aspiration unless atypical features appear or the effusion fails to respond to therapy. 1, 2

Key Historical and Physical Examination Features

• Document medication history carefully, as tyrosine kinase inhibitors and numerous other drugs cause exudative effusions 2
• Assess for occupational asbestos exposure in all cases 2
• Suspect pulmonary embolism when approximately 75% of patients present with pleuritic chest pain, the effusion occupies less than one-third of the hemithorax, and dyspnea is disproportionate to effusion size 1, 2
• Clinical assessment alone correctly identifies transudates in all cases when the clinical context is appropriate (e.g., decompensated heart failure with confirmatory chest radiograph) 1

Imaging Strategy

First-Line Imaging

• Obtain posteroanterior chest radiography as the initial imaging modality to determine laterality and detect moderate-to-large effusions (detects approximately 200 mL of fluid) 3
• Perform thoracic ultrasound on every patient at initial presentation before any pleural procedure—it is now considered an extension of the physical examination 2, 3

Ultrasound Assessment

• Ultrasound confirms fluid presence, determines if diagnostic aspiration is safe, and provides information on effusion size and character 2
• Look for diaphragmatic or parietal pleural nodularity as features suggesting malignancy 2
• Ultrasound detects effusions as small as 3-5 mL and identifies internal echoes indicating complicated effusions 3

Advanced Imaging

• Order contrast-enhanced CT of the chest (venous phase) when aspiration is unsafe, malignancy is suspected, or diagnosis remains unclear after initial evaluation 2, 3
• Perform CT with fluid present to better visualize the pleura and identify optimal biopsy sites 1, 2, 3

Indications for Diagnostic Thoracentesis

Perform thoracentesis for:

• Any unilateral pleural effusion 2
• Any effusion when the underlying diagnosis is uncertain 2
• Suspected parapneumonic effusion or empyema 2

Do NOT aspirate bilateral effusions that are clinically consistent with a transudate unless any of the following are present: atypical features, normal cardiac silhouette on chest radiograph, unilateral or asymmetric appearance, progressive enlargement, or lack of response to therapy 2, 4

Thoracentesis Technique and Fluid Analysis

Aspiration Technique

• Use a fine-bore 21-gauge needle attached to a 50 mL syringe for diagnostic sampling 1, 2, 3
• Ultrasound guidance is required for all thoracentesis procedures to maximize success and minimize complications 2, 3

Routine Pleural Fluid Analysis

Place samples in both sterile containers AND blood culture bottles to improve diagnostic yield 1, 2, 3

Analyze the following:

• Visual appearance and odor (the unpleasant aroma of anaerobic infection guides antibiotic choice) 1, 2
• Protein and lactate dehydrogenase (LDH) to differentiate transudate from exudate 1, 2, 3
• pH in all non-purulent effusions when infection is suspected 1, 2, 3
• Gram stain with aerobic/anaerobic cultures 1, 2, 3
• Acid-fast bacilli stain and tuberculosis culture 1, 2, 3
• Cytology 1, 2, 3
• Cell count with differential (lymphocyte predominance suggests malignancy or tuberculosis) 2, 3
• Pleural fluid hematocrit if the fluid appears bloody (for diagnosis of hemothorax) 1, 2

Classification: Transudate vs. Exudate

Apply the following algorithm:

• Pleural protein < 25 g/L → transudate 2
• Pleural protein > 35 g/L → exudate 2
• Pleural protein 25-35 g/L → apply Light's criteria:
  • Pleural/serum protein ratio > 0.5, OR
  • Pleural/serum LDH ratio > 0.6, OR
  • Pleural LDH > 2/3 of upper normal serum LDH
  • If any criterion is met → exudate 1, 2, 3

Management Based on Classification

Transudates

Treat the underlying condition (heart failure, cirrhosis, nephrotic syndrome) and reassess if there is no clinical improvement or if atypical features develop 2

Exudates Without Clear Diagnosis

• Obtain contrast-enhanced CT while fluid is still present to better visualize the pleura and select optimal biopsy sites 1, 2
• Obtain pleural tissue via ultrasound- or CT-guided biopsy, closed pleural biopsy, or thoracoscopy 1, 2
• Send tissue for histology and TB culture together with repeat pleural aspiration for cytology and microbiological studies 1, 2
• Recognize that cytology alone identifies only 60% of malignant effusions; if cytology is nondiagnostic, pleural tissue biopsy is required 1, 2, 3

Parapneumonic Effusions

• Pleural fluid pH < 7.2 indicates complicated parapneumonic effusion requiring prompt referral for catheter or chest-tube drainage with possible intrapleural fibrinolytic therapy or thoracoscopy 2, 4
• Simple parapneumonic effusions (pH > 7.2) generally resolve with antibiotic therapy alone 2

Critical Pitfalls to Avoid

• Do not reflexively tap all bilateral effusions when the clinical context strongly suggests a transudate, as this leads to unnecessary procedures 2
• Do not assume bilateral effusions are always benign—malignancy can present bilaterally 2
• Do not delay thoracentesis in patients with fever and an effusion, because early sampling is essential to prevent progression to empyema 2
• Maintain a high index of suspicion for pulmonary embolism in patients with pleural effusion, recognizing that pleural fluid studies are not diagnostic for embolism 1, 2
• Always use ultrasound guidance for thoracentesis to significantly reduce complication rates 2, 3
• Unilateral effusions in heart failure patients warrant investigation for alternative causes 5

Referral Indications

Refer to a chest physician when:

• Fluid analysis and clinical features have not yielded a diagnosis 1, 2
• Complicated parapneumonic effusion (pH < 7.2) is identified 2, 4
• Malignancy is suspected but cytology is nondiagnostic 2