A sample of pleural fluid is examined under the microscope. If organisms are detected by smear, other methods to specifically identify them may be performed.

It is important not to cough, breathe deeply, or move when the fluid sample is being taken. There is no other special preparation for the test.

The specimen is obtained by thoracentesis (a needle aspiration of fluid in the pleural space). An area on the chest is cleansed with antibacterial soap and numbed with local anesthetic. A needle is placed between the ribs, and a sample of fluid is withdrawn from the chest.

You may feel a stinging sensation when the anesthetic in injected. You may feel some pressure and slight localized pain when the thoracentesis needle enters the pleural space. A chest x-ray is usually done following the test to be sure the lung tissue was not affected by the test.

The test is performed when infection of the pleural space is suspected, or when an abnormal collection of pleural fluid is noticed by chest x-ray.

Normally, no organisms are present in the pleural fluid.

Positive results may indicate that microorganisms or cancer cells are present. Other tests can help identify the specific type of infection or cancer.

There is a risk of internal bleeding into the lung and pneumothorax (collapsed lung). Serious complications are extremely rare.

The fluid smear cost and the cost of the procedure to obtain the specimen are charged separately. Other tests may be done on the sample of fluid obtained.