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Pleural Fluid Analysis

What is a pleural fluid analysis?

A pleural fluid analysis is a group of tests that examine a sample of abnormal fluid that builds up in the space between your lungs and chest wall. This space is called the pleural space.

The pleura is a thin tissue that covers the outside of your lungs and lines the inside wall of your chest. Normally, a small amount of fluid, called pleural fluid, fills the space between the two layers of pleural tissue. This fluid keeps the pleura moist and helps the layers of tissue glide smoothly over each other as you breathe.

If too much fluid builds up in your pleural space, it's called a pleural effusion. Many different types of conditions can cause pleural effusion. The fluid buildup doesn't always cause symptoms, but if it presses on your lungs, you may have trouble breathing, even if your lungs are healthy.

A pleural fluid analysis can help find the cause of a pleural effusion. Treating the cause may help keep the fluid from building up in the future. The fluid buildup you have now usually needs treatment, too. Without treatment, it may lead to a collapsed lung and other serious health problems.

Other names: pleural fluid aspiration, diagnostic thoracentesis, pleural fluid tap, pleural fluid culture, pleural fluid cytology, pleural effusion testing, LDH body fluid

What is it used for?

A pleural fluid analysis helps find the cause of a pleural effusion. There are different types of fluid that can build up in the pleural space. The type of fluid you have will depend on the cause. A pleural fluid analysis is used to figure out which type of fluid you have. This information helps narrow down the possible causes.

A pleural fluid analysis alone may not find the specific condition causing your pleural effusion. But the results help your health care provider decide which other tests to order to help make a final diagnosis.

Why do I need a pleural fluid analysis?

You may need this test if you have symptoms of pleural effusion and an imaging test, such as an MRI scan or chest x-ray, shows that you have fluid buildup around your lungs.

The symptoms of pleural effusion depend on the cause and the amount of fluid. They may include:

  • Chest pain that doesn't have another explanation
  • Dry cough that doesn't bring up mucus
  • Trouble breathing
  • Fatigue

If the cause of your pleural effusion is known, you may not need a pleural fluid analysis even if you have symptoms.

Pleural effusions don't always cause symptoms, so sometimes a pleural effusion is found on a chest imaging test that's done for another reason. If this happens, a pleural fluid test is often done, even though you don't have symptoms.

What happens during a pleural fluid analysis?

Your provider will remove some pleural fluid using a hollow needle. This is done with a procedure called thoracentesis. The procedure may be done in a doctor's office or hospital. It may include these steps:

  • You may be asked to remove your clothes and put on a paper or cloth gown.
  • You may breathe oxygen through a face mask or short tubes that go into your nostrils.
  • You will sit up straight with your arms resting on a table. If you can't sit, you may lie on your side.
  • Your provider will clean an area on your back where the needle will be inserted.
  • You'll be given an injection (a shot) to numb the area so you won't feel any pain.
  • When the area is numb, your provider will insert a needle into your back between your ribs. The needle will go into the pleural space. Your provider may use ultrasound imaging to help see where to insert the needle.
  • You may feel some pressure when the needle goes in and when your provider removes the fluid.
  • At times, you may be asked to hold your breath during the procedure.
  • When enough fluid has been removed, the needle will be taken out and the fluid will be sent to a lab. A small bandage will be placed where the needle went into your back.
  • After the test, you may have a chest x-ray to check for any problems.

Will I need to do anything to prepare for the test?

In general, a thoracentesis doesn't require special preparations. Your provider will let you know if there's anything you need to do or avoid doing. For example, you may need to stop taking certain medicines before your test. But never stop taking any prescription medicines without talking with your provider first. or a blood test.

Are there any risks to the test?

Thoracentesis is a generally safe procedure. Serious complications are uncommon. They include:

  • A collapsed lung if the needle goes into your lung
  • Bleeding that causes blood to collect near the lung where the needle was put in
  • Pulmonary edema, a condition in which fluid builds up inside your lungs if too much pleural fluid is removed or if the fluid is removed too quickly
  • Infection if bacteria get into your body during the procedure
  • Injury to your liver or spleen if the needle goes into either organ

What do the results mean?

A pleural fluid analysis usually includes results from several tests done on your fluid sample. Your provider will consider all your test results, your symptoms, and your medical history to understand what your results mean. Ask your provider to explain what your test results say about your health.

In general, the results of your pleural fluid analysis will show which type of fluid is in your pleural space. There are two main types of fluid, and each type has different possible causes. To find out which type of fluid you have, your test usually uses a set of guidelines called Light's criteria.

Light's criteria tell providers how to compare the amount of certain proteins in your pleural fluid to the amount of those proteins measured in your blood. The results of the comparison will show whether your pleural fluid is transudate or exudate:

Exudate contains different substances depending on the condition that caused it. For example, exudate from an infection may have signs of pus, bacteria, or fungus. Exudate from cancer usually has cancer cells. A chest injury may cause exudate that includes blood.

To help find the specific cause of your pleural effusion, your provider will order other tests on your fluid sample to check for certain substances. The tests you have will depend on the most likely causes of your pleural effusion, based on all the information about your condition.

In certain cases, you may also need other types of tests, including blood and/or urine tests, imaging tests, a bronchoscopy, or a biopsy.

If you have questions about your test results, talk with your provider.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a pleural fluid analysis?

If your pleural effusion causes symptoms, your provider will remove enough fluid during a thoracentesis to relieve your discomfort. This helps treat the pleural effusion, but not the cause.

If your provider thinks the fluid may quickly build up again, a special tube, called a catheter, may be inserted when your pleural fluid sample is taken. The tube will be left in place to let the fluid drain out of your body. If pleural fluid stops building up, the tube may be removed.

Other treatments for pleural effusion will depend on how severe it is and what's causing it.


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The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.