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A more recent article on pleural effusion is available.

Am Fam Physician. 2014;90(2):99-104

Patient information: See related handout on pleural effusion, written by the authors of this article.

Author disclosure: No relevant financial affiliations.

Pleural effusion affects more than 1.5 million people in the United States each year and often complicates the management of heart failure, pneumonia, and malignancy. Pleural effusion occurs when fluid collects between the parietal and visceral pleura. Processes causing a distortion in body fluid mechanics, such as in heart failure or nephrotic syndrome, tend to cause transudative effusions, whereas localized inflammatory or malignant processes are often associated with exudative effusions. Patients can be asymptomatic or can present with cough, dyspnea, and pleuritic chest pain. Dullness to percussion on physical examination suggests an effusion; chest radiography can confirm the diagnosis. Thoracentesis may be indicated to diagnose effusion and relieve symptoms. Ultrasound guidance is preferred when aspirating fluid. Routine assays for aspirated fluid include protein and lactate dehydrogenase levels, Gram staining, cytology, and pH measurement. Light's criteria should be used to differentiate exudative from transudative effusions. Additional laboratory assays, bronchoscopy, percutaneous pleural biopsy, or thoracoscopy may be required for diagnosis if the initial test results are inconclusive.

More than 1.5 million persons develop pleural effusions each year in the United States.1 Many of the disease processes commonly seen in primary care are associated with pleural effusion, which requires family physicians to be familiar with its causes, diagnosis, and management.

Clinical recommendationEvidence ratingReferencesComments
Thoracentesis should be performed with ultrasound guidance.A10, 20, 24 Ultrasonography increases the likelihood of successful aspiration, decreases the risk of organ puncture (odds ratio for pneumothorax with ultrasonography = 0.3 to 0.8), and is associated with lower hospital costs.
Light's criteria should be used to differentiate transudative from exudative effusions.C10, 27, 28 Light's criteria have a diagnostic accuracy of 93% to 96%.
In patients with a pleural effusion classified as exudative by Light's criteria in which a cardiac etiology is suspected, N-terminal pro-brain natriuretic peptide can help differentiate cardiac from noncardiac conditions.C10, 11

Etiology and Pathogenesis

The visceral and parietal pleural membranes border a potential space within the thoracic cavity. Normally, a small physiologic amount of pleural fluid (0.1 mL per kg) rests within this space. Oncotic and hydrostatic pressures regulate fluid movement between the pleura, which adapt to a range of pressures to maintain the amount of fluid within a preset range. Abnormally high capillary and interstitial hydrostatic pressures can cause an abnormal accumulation of pleural fluid (e.g., in heart failure), as can an abnormally decreased capillary oncotic pressure (e.g., in nephrotic syndrome). Fluid that accumulates as a result of an imbalance in these forces produces transudative effusions. Additionally, inflammatory and malignant processes can promote local capillary and pleural membrane permeability or lymphatic blockage, which allows for the accumulation of exudative pleural fluid (i.e., fluid that is higher in protein and lactate dehydrogenase than transudative fluid).2 Furthermore, an interruption in diaphragmatic integrity can allow fluid to enter the pleural space.3

Pleural effusions can arise from a variety of disease states or medications (Table 1114 and Table 215 ). Despite the variety of conditions associated with effusions, many are idiopathic; these effusions tend to follow a benign course.16

ConditionExudative or transudativeClinical clues
Most common (by decreasing frequency)
Heart failureTransudativeHypoxia, pulmonary/peripheral edema
Bacterial pneumoniaExudativeChills, cough, fever, infiltrate
Pulmonary embolismExudativeDyspnea, immobilization, pleuritic chest pain, recent travel
MalignancyExudativeHistory of cancer, lung mass
Viral diseaseExudativeCough, fatigue, fever, muscle aches, rash
Post-cardiac surgeryExudativeRecent surgery
Less common (alphabetical order by organ system)
PericarditisExudativeElectrocardiographic findings, pericardial effusion on ultrasonography, sharp chest pain
Pulmonary vein stenosisExudativeRecent heart catheterization
Superior vena cava obstructionTransudativeFacial swelling and ruddy complexion, upper extremity swelling
Abdominal abscessExudativeAbdominal pain, chills, fever, nausea, vomiting
CirrhosisTransudativeHistory of alcohol abuse or viral hepatitis; ascites, caput medusae, palmar erythema
Esophageal perforationExudativeHistory of esophageal tumor or reflux; chest or abdominal pain, fever
PancreatitisExudativeAbdominal pain, anorexia, elevated amylase and lipase levels, nausea, vomiting
Post-abdominal surgeryExudativeRecent surgery
EndometriosisExudativeDysmenorrhea, infertility, pelvic pain
Meigs syndromeExudativeHistory of ovarian tumor
Ovarian hyperstimulation syndromeExudativeHistory of infertility treatment, abdominal pain
Postpartum effusionExudativeRecent childbirth
UrinothoraxTransudativeRecent urologic procedure, urinary obstruction
MesotheliomaExudativeHistory of asbestos exposure, pleural mass
ChylothoraxExudativeChest mass, lipids in pleural fluid, trauma
PseudochylothoraxExudativeHistory of tuberculosis or pleural disease, lipids in pleural fluid, rheumatoid disease
Medications (Table 2)ExudativeMedication use
Nephrotic syndromeTransudativeEdema, proteinuria
Rheumatoid arthritisExudativeJoint pain and swelling
Yellow nail syndromeExudativeLymphedema, yellow nails
Beta blockers
Ergot alkaloids
Nitrofurantoin (Furadantin)
Phenytoin (Dilantin)

Clinical Presentation

Patients with pleural effusion can be asymptomatic or can present with dyspnea, cough, or pleuritic chest pain. The history and physical examination can narrow the diagnostic considerations (Table 3110,13 and Table 417 ). The history should focus on differentiating pulmonary etiologies from cardiovascular and other causes of effusion. A thorough chest examination should be performed, with particular attention to dullness to percussion because it is sensitive and specific for diagnosing effusion.17 Figure 1 outlines an approach to evaluating and diagnosing the cause of pleural effusion.10,1720

Signs and symptomsSuggested etiology
Distended neck veinsHeart failure, pericarditis
Dyspnea on exertionHeart failure
FeverAbdominal abscess, empyema, malignancy, pneumonia, tuberculosis
HemoptysisMalignancy, pulmonary embolism, tuberculosis
OrthopneaHeart failure, pericarditis
Peripheral edemaHeart failure
S3 gallopHeart failure
Unilateral lower extremity swellingPulmonary embolism
Weight lossMalignancy, tuberculosis
FindingSensitivity (%)Specificity (%)
Pleural friction rub5.399
Asymmetric chest expansion7491
Reduced vocal resonance7688
Reduced vocal fremitus8286
Auscultatory percussion30 to 9684 to 95
Diminished breath sounds42 to 8883 to 90
Dullness to percussion30 to 9081 to 98


When pleural effusion is suspected, chest radiography should be performed to confirm the diagnosis. Abnormal findings can be detected on posteroanterior radiography in the presence of 200 mL of fluid, and on lateral radiography with as little as 50 mL of fluid.10 Lateral decubitus radiography may be obtained to help determine the size of the effusion and whether it is free-flowing or loculated.

If chest radiography is inconclusive, computed tomography and ultrasonography may be useful.21,22 Computed tomography can detect effusions not apparent on plain radiography, distinguish between pleural fluid and pleural thickening, and provide clues to the underlying etiology.22 Ultrasonography is more accurate than auscultation or chest radiography in detecting pleural effusion in the critical care setting, and is more sensitive than computed tomography in detecting pleural fluid septations.10,21



Diagnostic small-volume aspiration of pleural fluid (50 to 60 mL) is indicated when the underlying cause of effusion is unknown. Large-volume aspiration is reserved for treatment of effusion-related symptoms, such as dyspnea.10,23 Emergent thoracentesis and/or chest tube placement is necessary in patients with pleural effusion and significant respiratory or cardiac decompensation.

Chest radiography can help guide patient selection. Aspiration is required in an undiagnosed patient with an effusion larger than 1 cm on a decubitus film.18 Likewise, an effusion larger than 5 cm on a lateral radiograph in a patient with pneumonia warrants diagnostic aspiration, because parapneumonic effusions and empyema can cause nonresponse to treatment.19 Patients with suspected transudative bilateral effusions should not undergo thoracentesis unless they have atypical features (e.g., fever, pleuritic chest pain, effusions of disparate size) or do not respond to treatment.10,18 Guidelines recommend that, when possible, thoracentesis be performed with ultrasound guidance; this increases the likelihood of successful aspiration, decreases the risk of organ puncture (odds ratio for pneumothorax with ultrasonography = 0.3 to 0.8), and is associated with lower hospital costs.10,20,24 A recent systematic review, however, indicates no benefit from skin marking or ultrasound-guided needle insertion.25 Postprocedural chest radiography is not indicated unless symptoms develop.20,26 A video depicting thoracentesis is available at (subscription required).


Gross appearance of the pleural fluid can provide diagnostic clues (Table 5).10,11 Milky fluid may indicate a chylothorax or pseudochylothorax, whereas food particles suggest an esophageal perforation. Routine testing includes protein and lactate dehydrogenase levels, Gram staining and culture, cytology (malignant effusions can be diagnosed by cytology in 60% of cases), and pH level. Glucose levels may also be obtained.10,18

FindingPotential etiology
Anchovy brown fluidRuptured amoebic abscess
Bile stainingCholothorax (i.e., biliary fistula)
Black fluidAspergillus infection
Food particlesEsophageal perforation
Milky fluidChylothorax or pseudochylothorax
Putrid odorAnaerobic empyema

Protein and lactate dehydrogenase levels help determine whether collected pleural fluid represents a transudative or exudative effusion and are used to assess for Light's criteria (Figure 1).10,1720 Light's criteria are 99.5% sensitive for diagnosing exudative effusion27 and differentiate exudative from transudative effusions in 93% to 96% of cases.10,28 In the absence of serum testing, pleural fluid protein and lactate hydrogenase levels have a 92% concordance with Light's criteria for differentiating between transudative and exudative effusions.29 A recent systematic review revealed that a pleural cholesterol level greater than 55 mg per dL (1.42 mmol per L), a pleural to serum cholesterol ratio greater than 0.3, and a pleural lactate dehydrogenase level greater than 200 units per L (3.3 μkat per L) were among the most specific findings for diagnosing an exudate.25

Gram staining may help identify a causative pathogen. A cell count may also reveal an underlying etiology. Neutrophil predominance tends to indicate an acute process, such as a parapneumonic effusion or pulmonary embolism, whereas lymphocyte predominance may be noted in longstanding effusions, heart failure, malignancy, tuberculosis, and thoracic duct injury.10,30 Pleural fluid pH less than 7.30 may indicate a malignant effusion, connective tissue disease, or esophageal perforation; a value less than 7.20 indicates the need for tube drainage in patients with parapneumonic effusions, especially in the setting of an elevated lactate dehydrogenase level and a glucose level less than 60 mg per dL (3.3 mmol per L).10 Further tests are guided by clinical suspicion and may include acid-fast bacillus testing (including adenosine deaminase) for tuberculosis and measurement of triglyceride, cholesterol, amylase, hematocrit, and N-terminal pro-brain natriuretic peptide levels (Table 6).6,10,11,18,30,31 Tumor markers are not routinely obtained.

Acid-fast bacillus, adenosine deaminase level, Mycobacterium tuberculosis cultureIndicated if tuberculosis is a concern; measurement of adenosine deaminase may also be useful in determining the presence of tuberculosis (sensitivity and specificity > 90%, although it may also be elevated in patients with empyema or malignancy)
Amylase levelElevated in patients with pancreatitis; may also be elevated in those with malignancy, esophageal perforation, or tuberculosis
Hematocrit levelHematocrit > 1% indicates possible pneumonia, pulmonary embolism, malignancy, or trauma; pleural fluid hematocrit > 0.5 × peripheral blood hematocrit indicates hemothorax
N-terminal pro-brain natriuretic peptide levelElevated in patients with heart failure; useful in diagnosing heart failure when effusion is classified as exudative by Light's criteria
pH and glucose levelspH < 7.20 and glucose < 60 mg per dL (3.3 mmol per L) may indicate a complicated parapneumonic effusion or empyema; chest tube draining may be indicated
Triglyceride and cholesterol levelsHelpful in diagnosing and differentiating chylothorax and pseudochylothorax (patients with pseudochylothorax have increased cholesterol and decreased triglyceride levels)
Tumor markersMay be ordered based on clinical suspicion; includes carcinoembryonic antigen, cancer antigen 125, cancer antigen 15-3, cytokeratin 19 fragment, and mesothelin testing

If thoracentesis is unsuccessful or the results of fluid analysis are unclear, pulmonary consultation and additional testing can be helpful. Percutaneous pleural biopsy or thoracoscopy may be indicated if malignancy is suspected. Bronchoscopy may be warranted if hemoptysis or bronchial obstruction is present.10

Data Sources: A PubMed search was completed using the keyword and medical subject headings pleural effusion and thoracentesis. The search included randomized controlled trials, meta-analyses, clinical trials, systematic reviews, clinical practice guidelines, and review articles. Also searched were Essential Evidence Plus, the National Guideline Clearinghouse, and the Cochrane Database of Systematic Reviews. Search dates: January 2012 through April 2014.

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