What is the most common cause of a right-sided pleural effusion?

Most Common Cause of Right-Sided Pleural Effusion

Heart failure is the most common cause of right-sided pleural effusion, accounting for more than 80% of transudates, and when unilateral, heart failure effusions preferentially occur on the right side. 1, 2, 3

Primary Etiologies by Frequency

The overall distribution of pleural effusion causes includes:

• Heart failure: 29% - This represents the single most common etiology and accounts for >80% of all transudative effusions 1
• Malignancy: 26% - Lung cancer is the leading malignant cause, followed by breast cancer 1
• Pneumonia (parapneumonic effusions): 16% 1
• Tuberculosis: 6% 1
• Cirrhosis (hepatic hydrothorax): 3% - Accounts for ~10% of transudates and characteristically appears on the right side due to diaphragmatic defects 1, 2

Why Right-Sided Predominance Occurs

Several conditions show right-sided preference:

• Heart failure effusions, when unilateral, are more commonly right-sided 2, 3
• Hepatic hydrothorax characteristically appears on the right (90% of cases) due to diaphragmatic defects allowing ascitic fluid passage 2
• Benign asbestos pleural effusions occur on the right in 69-76% of cases 4

Critical Diagnostic Considerations

Transudate vs Exudate Framework

For right-sided effusions, first determine if transudate or exudate:

• Transudates (heart failure, cirrhosis, nephrotic syndrome) require treating the underlying condition 1
• Exudates (malignancy, infection, pulmonary embolism) require specific diagnostic workup 1

NT-proBNP as Diagnostic Tool

Serum or pleural fluid NT-proBNP ≥1,500 pg/mL accurately diagnoses heart failure as the primary or contributory cause 1, 2

This is particularly valuable when:

• Light's criteria misclassify a cardiac transudate as an exudate (occurs in 25-30% of cases) 1, 5
• Multiple etiologies may be present 2

High-Risk Exudative Causes Not to Miss

When the effusion is exudative, prioritize these diagnoses:

1. Malignancy - Lung cancer most common, presents ipsilateral to tumor 2
2. Parapneumonic effusion/empyema - Look for fever, leukocytosis, adjacent consolidation on CT 2
3. Pulmonary embolism - Present in 40% of PE cases, typically small and ipsilateral to embolus, with pleuritic pain in 75% 5, 2
4. Tuberculosis - Must reconsider in persistent undiagnosed cases as it is amenable to specific treatment 4, 5

Common Pitfalls to Avoid

• Do not assume bilateral distribution rules out heart failure - Unilateral right-sided effusions occur with heart failure 3
• Do not rely solely on Light's criteria - Use serum-effusion albumin gradient >1.2 g/dL or NT-proBNP when heart failure is suspected but Light's criteria suggest exudate 1
• Do not overlook hepatic hydrothorax - Consider in any patient with cirrhosis and right-sided effusion, even without obvious ascites 2
• Do not delay thoracentesis - Diagnostic sampling is essential for unexplained effusions to differentiate between treatable causes 2