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Pleural fluid sampling (or thoracentesis)

Medically reviewed by Last updated on Mar 28, 2023.

What is the test?

Harvard Health Publishing

Some infections and diseases cause fluid to accumulate in the space between the lung and the rib cage or between the lung and the diaphragm. This collection of fluid is called a pleural effusion. A pleural effusion might be detected on a chest x-ray or chest CT scan. Sampling this fluid is important because it enables doctors to understand what caused the fluid to collect and how to treat the problem. The fluid can be sampled with a needle.

How do I prepare for the test?

You will need to sign a consent form giving your doctor permission to perform this test. This test can be performed in the doctor's office or if the patient is in the hospital at the bedside. Generally your doctor will decide whether you need to be in the hospital based on your medical condition. A chest x-ray or an ultrasound is done before the procedure.

Tell your doctor if you have ever had an allergic reaction to lidocaine or a similar numbing medicine used most often at the dentist's office. If you take medicines that affect blood clotting, talk with your doctor. It may be necessary to stop or adjust the dose of these medicines before your test.

What happens when the test is performed?

You wear a hospital gown and sit on a bed or table leaning forward against some pillows. The doctor listens to your lungs with a stethoscope and may tap on your back to find out how much fluid has collected.

An antiseptic solution is used to disinfect an area of skin on one side of your back. A small needle is used to numb a patch of skin between two of your lower ribs. The numbing medicine usually stings for a second.

A needle on an empty syringe is then inserted into the skin and pushed forward between the ribs. The needle is advanced until it enters the fluid collection inside your chest wall. You might feel some minor pressure as the needle is inserted.

The syringe draws out a fluid sample. If your doctor wants to remove a larger amount of fluid, a thin, soft plastic tube is used instead. The tube leads to a large jar. While the doctor is attaching the tubing, he or she might ask you to hum out loud. This humming is for your safety: It prevents you from taking a deep breath, which could expand your lung, causing it to touch the needle.

It sometimes takes 15 minutes or longer to remove the necessary amount of fluid. Most patients feel no discomfort during this time, although a few patients feel some chest pain at the end of the procedure as their lung expands and touches the chest wall. After the fluid is removed, a bandage is placed on your back.

What risks are there from the test?

This procedure carries a few serious risks, but most patients have no complications. If the needle touches the lung it may create an air leak, which is seen on the x-ray and might require you to stay in the hospital for a few days. Some patients with this complication need to have a plastic tube (called a chest tube) inserted between two ribs. The tube uses vacuum pressure to keep the lung expanded until it has healed.

Other risks include bleeding into the fluid space or infection. Rarely, if a large amount of fluid is removed (more than one liter); the rapid pressure changes in your lung as it fills this space can cause some fluid to seep from your bloodstream into your lungs. This is called pulmonary edema. Let your doctor know if you feel shortness of breath following the procedure.

Must I do anything special after the test is over?

You will need to have an x-ray taken after the sampling is completed. Your breathing should feel the same (or better) after the procedure.

How long is it before the result of the test is known?

The fluid may be tested for a variety of things, including infection and cancer. Cells in the fluid will be examined. It may be several days before full results are available.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.