Pleural fluid analysis examines fluid that has collected in the pleural space -- the small area between the membranes that line the lungs.

See also: Pleural effusion

A procedure called thoracentesis is used to get a sample of pleural fluid. The health care provider examines the sample to look for:

• Cancerous (malignant) cells
• Cellular makeup
• Chemical content
• Tiny organisms that can cause disease (microorganisms)

The test is no more invasive than having blood drawn. There is no special preparation. Do not cough, breathe deeply, or move during the test to avoid injury to the lung. You may have a chest x-ray before or after the test.

You will sit on the edge of a chair or bed with your head and arms resting on a table. The health care provider will clean the skin around the insertion site and drape the area. A local pain-killing medicine (anesthetic) is injected into the skin, which stings a bit, but only for a few seconds.

The thoracentesis needle is inserted above the rib into the pocket of fluid. As fluid drains into a collection bottle, many people cough a bit as the lung reexpands to fill the space where fluid had been. This sensation normally lasts for a few hours after the test is completed. Tell your health care provider if you have sharp chest pain or shortness of breath.

The test is performed to determine the cause of a pleural effusion, and to relieve the shortness of breath that a large pleural effusion can cause.

Normally the pleural cavity contains less than 20 milliliters (4 teaspoons) of clear, yellowish (serous) fluid.

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

Abnormal results may indicate possible causes of pleural effusion, such as:

• Cancer (neoplasm)
• Cirrhosis
• Heart failure
• Infection

If the health care provider suspects an infection, a culture of the fluid is done to check for microorganisms.

Another condition under which the test may be performed is hemothorax, a collection of blood in the pleura.

The risks of thoracentesis are:

• Collapse of the lung (pneumothorax)
• Excessive loss of blood
• Fluid re-accumulation
• Infection
• Pulmonary edema
• Respiratory distress

Serious complications are uncommon.