Pleural Effusion

How to Cite This Chapter: Wongkarnjana A, Hambly N, Sładek K, Jankowski M. Pleural Effusion. McMaster Textbook of Internal Medicine. Kraków: Medycyna Praktyczna. Accessed June 18, 2024.
Last Updated: February 13, 2022
Last Reviewed: February 13, 2022
Chapter Information

Definition and Clinical FeaturesTop

Pleural effusion results from an imbalance between fluid formation and resorption within the pleural space. It may be transudative or exudative. Pleural effusion may be asymptomatic or manifest with dyspnea, trepopnea, orthopnea, cough, or chest pain, depending the on underlying conditions and degree of fluid accumulation. Signs of pleural effusion and differential diagnosis: see Table 1 in Respiratory Sounds. Diagnosis is based on results of imaging studies (chest radiographs, computed tomography [CT], ultrasonography) and analyses of pleural fluid. Also see Thoracentesis.


If history and physical examination are suggestive of pleural effusion (decreased breath sound with dullness on percussion), further investigations are required to confirm the diagnosis. Posteroanterior chest radiography should be performed in the assessment of pleural effusion. Pleural effusions ≥200 mL normally exhibit an abnormal opacity with decreased lung volume on chest radiographs, but effusions of as little as 50 mL can cause blunting of the costophrenic angle. To date, ultrasonography is the gold standard investigation to diagnose a pleural effusion. Ultrasonography detects fibrin and septations within the pleural space with greater sensitivity than CT. It also increases the success rate of thoracentesis while minimizing procedure-related complications. Chest CT with contrast is useful in the evaluation of exudative pleural effusions, providing critical findings that may be suggestive of a malignant effusion, mesothelioma, complications of lung infection (parapneumonic effusion, empyema), and vascular or lymphatic obstruction.

Pleural fluid analysis is critical in characterizing the type of pleural effusion and guiding further investigations. Pleural effusion may appear serous, serosanguinous, bloody, purulent, or milky. Some presentations are associated with specific conditions, such as putrid purulent fluid in anaerobic empyema, milky fluid in chylothorax or pseudochylothorax, and bile-stained fluid in biliary-pleural fistula. Results of pleural fluid analysis are used to categorize pleural effusion as transudative and exudative using Light’s criteria (Table 1), which are highly sensitive for an exudative process (sensitivity, 98%; specificity, 83%). Pleural fluid differential cell counts are helpful in the differential diagnosis but not disease-specific (Table 2). Neutrophil-predominant effusions are associated with acute processes, while effusion related to malignancy, tuberculosis, and cardiac failure are commonly lymphocytic. However, any long-standing pleural effusion over time tends to become populated by lymphocytes. Fluid cytology should be done in all patients with exudative effusions to evaluate for malignancy. Gram stain and culture should be considered for a newly diagnosed effusion.


Table 17.16-1. Light’s criteria for distinguishing exudative from transudative pleural effusion

Light’s criteria

Sensitivity for exudate

Specificity for exudate

Exudative pleural effusion if meets ≥1 of:




Pleural fluid protein to serum protein ratio >0.5



Pleural fluid LDH to serum LDH ratio >0.6



Pleural fluid LDH >2/3 of ULN for serum LDH



Adapted from N Engl J Med. 2002;346(25):1971-7.

LDH, lactate dehydrogenase; ULN, upper limit of normal.

Table 17.16-2. Pleural fluid differential cell counts and possible associated diseases

Neutrophil predominance

– Parapneumonic effusion

– Pulmonary embolism

– Early tuberculous pleuritis

– Benign asbestos pleural effusion

Lymphocyte predominance

– Malignancy

– Tuberculous pleuritis

– Lymphoma

– Cardiac failure

– After coronary bypass graft

– Rheumatoid effusion

– Chylothorax

– Uremic pleuritis

– Sarcoidosis

– Yellow nail syndrome

– Pulmonary embolism

Eosinophilia (≥10% of nucleated cells)

– Parapneumonic effusion

– Drug-induced pleuritis

– Benign asbestos pleural effusion

– Eosinophilic granulomatosis with polyangiitis

– Lymphoma

– Pulmonary embolism

– Parasitic infestation

– Malignancy

Pleural eosinophilia is a relatively nonspecific finding.

Adapted from Thorax. 2010;65 Suppl 2:ii4-17.

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