Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Pleural Effusion


Richard W. Light

, MD, Vanderbilt University Medical Center

Last full review/revision Jan 2021| Content last modified Jan 2021
Click here for the Professional Version
Topic Resources

Pleural effusion is the abnormal accumulation of fluid in the pleural space (the area between the two layers of the thin membrane that covers the lungs).

  • Fluid can accumulate in the pleural space as a result of a large number of disorders, including infections, tumors, injuries, heart, kidney, or liver failure, blood clots in the lung blood vessels (pulmonary emboli), and drugs.

  • Symptoms may include difficulty breathing and chest pain particularly when breathing and coughing.

  • Diagnosis is by chest x-rays, laboratory testing of the fluid, and often computed tomography angiography.

  • Large amounts of fluid are drained with a tube inserted into the chest.

Normally, only a thin layer of fluid separates the two layers of the pleura. An excessive amount of fluid may accumulate for many reasons, including heart failure, cirrhosis, pneumonia, and cancer.

A large number of disorders may cause pleural effusion. Some of the more common causes (listed as most common to least common) include

Types of fluid

Depending on the cause, the fluid may be either

  • Rich in protein (exudate)

  • Watery (transudate)

Doctors use this distinction to help determine the cause. For example, heart failure and cirrhosis are common causes of watery fluid in the pleural space. Pneumonia, cancer, and viral infections are common causes of pleural effusion with an exudative fluid.

Blood in the pleural space (hemothorax) usually results from a chest injury. Rarely, a blood vessel ruptures into the pleural space when no injury has occurred, or a bulging area in the aorta (aortic aneurysm) leaks blood into the pleural space.

Pus in the pleural space (empyema) can accumulate when pneumonia or a lung abscess spreads into the space. Empyema may also complicate an infection due to chest wounds, chest surgery, rupture of the esophagus, or an abscess in the abdomen.

Lymphatic (milky) fluid in the pleural space (chylothorax) is caused by an injury to the main lymphatic duct in the chest (thoracic duct) or by a blockage of the duct by a tumor.

Urine in the pleural space (urinothorax) is uncommon and can accumulate if the tubes that drain urine from the kidneys (ureters) are blocked.

Fluid in the pleural space that contains excessive amounts of cholesterol results from a long-standing pleural effusion caused by a condition such as tuberculosis or rheumatoid arthritis.

Symptoms of Pleural Effusion

Many people with pleural effusion have no symptoms at all. The most common symptoms, regardless of the type of fluid in the pleural space or its cause, are

  • Shortness of breath

  • Chest pain

Chest pain is usually of a type called pleuritic pain (the term pleurisy is no longer or only rarely used). Pleuritic pain may be felt only when the person breathes deeply or coughs, or it may be felt continuously but may be worsened by deep breathing and coughing. The pain is usually felt in the chest wall right over the site of the inflammation or infection that caused the effusion. However, the pain may be felt also or only in the upper abdominal region or neck and shoulder, which is called referred pain (see figure What Is Referred Pain?). Pleuritic pain can be caused by disorders other than pleural effusion.

Pleuritic chest pain due to a pleural effusion may disappear as fluid accumulates. Large amounts of fluid can cause difficulty in expanding one or both lungs when breathing, causing shortness of breath.


Major Causes of Pleuritic Chest Pain

Diagnosis of Pleural Effusion

  • Chest x-ray and/or ultrasonography

  • Laboratory tests done on a sample of the fluid

  • Sometimes computed tomography (CT) angiography

A chest x-ray, which shows fluid in the pleural space, is usually the first step in making the diagnosis. However, small amounts of fluid may not be visible on a chest x-ray.

Ultrasonography may help doctors identify a small accumulation of fluid.

Doctors may do thoracentesis. In this procedure, a specimen of the fluid is removed for examination using a needle. The appearance of the fluid may help doctors determine its cause. Certain laboratory tests evaluate the chemical composition of the fluid and determine the presence of bacteria, including the bacteria that cause tuberculosis. The fluid specimen is also examined for the number and types of cells and for the presence of cancerous cells.

If these tests cannot identify the cause of the pleural effusion, other tests may be done.

CT angiography more clearly shows the lung and the fluid and may show evidence of pneumonia, a pulmonary embolus, a mediastinal mass, a lung abscess, or a tumor that could be causing fluid to accumulate. For CT angiography to be done, people need to hold their breath during scanning.

If a serious diagnosis still seems possible, doctors may insert a viewing tube into the chest (called thoracoscopy). Occasionally, doctors need to obtain a sample (biopsy) of the pleura and/or lung. In about 20% of people with pleural effusion, the cause is not obvious after initial testing, and in some people a cause is never found, even after extensive testing.

Treatment of Pleural Effusion

  • Treatment of the disorder causing pleural effusion

  • Drainage of large pleural effusions

Small pleural effusions may not require treatment, although the underlying disorder must be treated. Sometimes the person is given analgesics until the fluid is drained or drains away on its own.

Larger pleural effusions, especially those that cause shortness of breath, may require drainage. Usually, drainage dramatically relieves shortness of breath. Often, fluid can be drained using thoracentesis. An area of skin between two lower ribs is anesthetized, then a small needle is inserted and gently pushed deeper until it reaches the fluid. A thin plastic catheter is often guided over the needle into the fluid to lessen the chance of puncturing the lung and causing a pneumothorax. Although thoracentesis is usually done for diagnostic purposes, doctors can safely remove as much as about 5 quarts (5 liters) of fluid at a time using this procedure.

When larger amounts of fluid must be removed, a tube (chest tube) may be inserted through the chest wall. After numbing the area by injecting a local anesthetic, doctors insert a plastic tube into the chest between two ribs. Then doctors connect the tube to a water-sealed drainage system that prevents air from leaking into the pleural space. A chest x-ray is taken to check the tube’s position. Drainage can be blocked if the chest tube is incorrectly positioned or becomes kinked. If the fluid is very thick or full of clots, it may not flow out.

Effusions caused by pneumonia

When an accumulation of fluid is due to pneumonia, intravenous antibiotics are needed. Doctors also typically remove a sample of the fluid for examination and testing. If the fluid is pus or has certain other characteristics, the fluid needs to be drained, usually with a chest tube. If the fluid is partitioned into separate compartments by scars that have developed within the pleural space, drainage is more difficult. Sometimes drugs called fibrinolytic drugs plus a drug that can help thin effusions that are thick (dornase alfa) are instilled into the pleural space to help drainage, which may avoid the need for surgery. (To be effective, both fibrinolytic drugs and dornase alfa must be used.)

If surgery is needed, it can be done by using a procedure called video-assisted thoracoscopic debridement or by making an incision through the chest wall (thoracotomy). During surgery, any thick peels of fibrous material over the lung surface are removed to allow the lung to expand normally.

Effusions caused by cancers

Fluid accumulation caused by cancers of the pleura may be difficult to treat because fluid often reaccumulates rapidly. Draining the fluid and giving antitumor drugs sometimes prevents further fluid accumulation. A small tube can be left in the chest so that the fluid can be drained periodically into vacuum bottles. But if fluid continues to accumulate, sealing the pleural space (pleurodesis) may be helpful. For pleurodesis, all fluid is drained through a tube, which is then used to administer a pleural irritant, such as a doxycycline solution, bleomycin, or a talc mixture, into the space. The irritant seals the two layers of pleura together, so that no room remains for additional fluid to accumulate. Pleurodesis can also be done using thoracoscopy.


Treatment of chylothorax focuses on eliminating the leakage from the lymphatic duct. Such treatment may consist of surgery, chemotherapy, or radiation therapy for a tumor that is blocking lymph flow.

Drugs Mentioned In This Article

Generic Name Select Brand Names
No US brand name
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Test your knowledge

Pulmonary-Renal Syndrome
Pulmonary-renal syndrome affects both the lungs and kidneys. This disorder causes bleeding into the lungs (diffuse alveolar hemorrhage) along with damage to the microscopic blood vessels in the kidney (glomerulonephritis). Which of the following is almost always the cause of this syndrome?
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID