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

Most Common Cause of Right-Sided Exudative Pleural Effusion

Malignancy is the most common cause of right-sided exudative pleural effusion, with lung cancer being the leading malignant etiology, followed by breast cancer. 1, 2

Primary Etiologies of Exudative Effusions

The differential diagnosis depends critically on patient age and geographic context:

Malignancy (Leading Cause Overall)

• Malignancy accounts for 42-77% of all exudative pleural effusions 1
• Lung carcinoma represents approximately one-third of all malignant effusions and is the single most common malignant cause 1, 2
• Breast carcinoma is the second most common malignancy, representing 3-27% of malignant effusions 1, 2
• Lymphomas (Hodgkin's and non-Hodgkin's) account for approximately 10-22% of malignant pleural effusions and characteristically present with lymphocyte-predominant exudates 1, 2
• Most types of pleural effusions, including malignant ones, show a preference for the right side 3

Tuberculosis (Geographic Variation)

• In regions with high TB incidence, tuberculosis is the most frequent cause (44.1% of exudative effusions), surpassing malignancy (29.6%) 3
• TB effusions occur predominantly in younger patients (mean age 39.7 years) and are most common in the first five decades of life, accounting for 69.8% of effusions in this age range 3

Critical Diagnostic Algorithm

Initial Evaluation

• Perform thoracic ultrasound on every patient at initial presentation to assess safety of diagnostic aspiration, effusion size, and character 4
• Look for nodularity of the diaphragm and parietal pleura on ultrasound, which are highly suggestive of malignancy 4

Diagnostic Thoracentesis

• Send pleural fluid for nucleated cell count with differential, total protein, LDH, glucose, pH, and cytology 2
• Apply Light's criteria first to confirm exudative nature (sensitivity 98%, specificity 72%) 1

When Malignancy is Suspected

• Cytology achieves approximately 80% diagnostic yield in malignancy but only 31-55% in lymphoma 1
• If initial cytology is negative but malignancy remains suspected, proceed to thoracoscopy, which has superior diagnostic yield (85% sensitivity for lymphoma with chromosome analysis) 1, 2
• CT chest/abdomen/pelvis should be obtained if malignancy is suspected 4

Key Clinical Clues

• Absence of contralateral mediastinal shift despite massive effusion suggests mediastinal fixation by tumor, mainstem bronchus occlusion, or extensive pleural involvement 2
• Hemoptysis with pleural effusion is highly suggestive of bronchogenic carcinoma 2
• History of asbestos exposure (pleural plaques on CT) should raise suspicion for mesothelioma 2
• Dull, aching chest pain (rather than pleuritic) with massive effusion suggests mesothelioma 2

Critical Pitfalls to Avoid

• Do not assume bilateral effusions exclude malignancy—malignant effusions can be bilateral 2
• Always obtain detailed occupational history including asbestos exposure when investigating all pleural effusions 4
• Review medication history carefully—tyrosine kinase inhibitors are now the most common drugs causing exudative pleural effusions 4
• In persistent undiagnosed effusions, reconsider tuberculosis and pulmonary embolism, as both are amenable to specific treatment 1
• Pleural fluid ANA testing is not helpful in diagnosing SLE as it merely mirrors serum levels 1

Age-Specific Considerations

• Patients under 50 years: Consider tuberculosis first, especially if lymphocyte-predominant 3
• Patients over 50 years: Malignancy becomes increasingly likely, with 74.5% of malignant effusions occurring in this age group 3