Test Quick Guide

Pleural fluid is found in the thin space between the lungs and the tissue covering the lungs. Pleural fluid testing involves removing a sample of this fluid so that it can be analyzed using one or more laboratory methods.

Testing is used to diagnose the cause of an abnormal buildup of pleural fluid, which is called a pleural effusion. The fluid is obtained by inserting a needle into the pleural space, a procedure known as thoracentesis.

About the Test

Purpose of the test

The purpose of testing pleural fluid is to determine the cause of a pleural effusion. A pleural effusion occurs when excess fluid collects in the area between the lungs and the tissue that covers the lungs, known as the pleural space.

While it is normal to have a small amount of fluid in the pleural space, a pleural effusion involves an abnormal buildup of fluid that can disrupt breathing and trigger other symptoms.

Pleural fluid testing plays an important role in diagnosing the reason why a pleural effusion has occurred. Many different health conditions can cause a pleural effusion, and identifying the underlying cause through pleural fluid testing can help guide treatment.

The procedure to obtain a sample of pleural fluid is known as thoracentesis, and it involves inserting a thin needle into the pleural space to withdraw a small amount of fluid. When thoracentesis is done for pleural fluid testing, it is called diagnostic thoracentesis. If the procedure is done to reduce fluid buildup and relieve symptoms, it is called therapeutic thoracentesis.

What does the test measure?

Pleural fluid testing may involve multiple different analyses and measurements of the same test sample. Common components of a pleural fluid test include:

  • Visual examination: Looking at the pleural fluid with the naked eye can detect abnormalities in its color, texture, and viscosity.
  • Odor analysis: A health professional can observe if there is any smell coming from the pleural fluid.
  • Microscopic evaluation: A sample of the pleural fluid can be placed on a laboratory slide and examined with a microscope. This can show the kinds of cells present, including germs like bacteria or fungi that can cause infections. A technique called a Gram stain applies a chemical dye to the slide, which can help reveal certain types of germs.
  • Cytology: Cytology is a detailed examination of the specific kinds of cells in the sample and is often utilized to determine whether cancer cells are present in the pleural fluid.
  • Bacterial or fungal culture: A culture test places a sample of the pleural fluid in a dish in the laboratory where it is left for a period of days to see if any bacteria or fungi grow.
  • Acidity: This measurement assesses the acidity, or pH, of the pleural fluid.
  • Cell count: This analysis determines the total number of cells in the sample and can also include a cell count differential, which calculates the amounts of each cell type that is present.
  • Total protein: Total protein is the sum of all types of proteins in the pleural fluid and may be reported as the concentration of protein in the sample.
  • Lactate dehydrogenase (LDH): Lactate dehydrogenase is a type of protein called an enzyme. It is involved in normal cell activity but may be produced at higher levels in response to inflammation or tissue damage.
  • Glucose: Glucose is a primary source of energy for cells. Glucose enters the body from the diet and is transported by the blood to cells throughout the body.

Some substances in pleural fluid are selected for evaluation based on a patient’s individual situation and health history. These components may include:

  • Cholesterol: Cholesterol is a waxy substance that is transported in the blood and helps nearly all cells in the body work properly.
  • Triglycerides: Triglycerides are a type of fat that is generated when extra calories are stored in cells to be used for future energy needs.
  • Creatinine: Creatinine is a chemical byproduct of muscle activity. Creatinine is normally removed from the body by the kidneys and urinary system.
  • Amylase: Amylase is a type of enzyme involved in the digestion of carbohydrates.
  • Adenosine deaminase (ADA): Adenosine deaminase is an enzyme that helps enable the proper function of certain kinds of immune cells.
  • N-terminal pro-brain natriuretic peptide (NT-proBNP): NT-proBNP is a type of protein that is associated with heart function.

A health care provider can explain which test components were ordered and the purpose of each component in evaluating the cause of an abnormal buildup of pleural fluid.

When should I get a pleural fluid test?

Pleural fluid testing is normally ordered if you are found to have a pleural effusion, which is usually after an x-ray, CT scan, or ultrasound shows a large buildup of fluid in the pleural space. These imaging tests may be performed if you have symptoms of a pleural effusion such as unexplained chest pain, shortness of breath, cough, or other breathing problems.

While most people with a pleural effusion will have a thoracentesis procedure for pleural fluid analysis, testing may not be necessary if the cause is already known or if the pleural effusion is small and not causing any symptoms.

Most people with a pleural effusion can safely undergo thoracentesis, but this procedure may not be recommended if you have certain coexisting conditions, such as uncontrollable coughing, serious lung disease, or a skin infection around where the needle would be inserted.

Finding a Pleural Fluid Test

How to get tested

A pleural fluid test is performed with a procedure called thoracentesis that allows fluid to be withdrawn from the pleural space after inserting a needle through your back.

Thoracentesis and pleural fluid analysis are ordered by a doctor and conducted by trained health professionals. The procedure can be performed in a doctor’s office, hospital, or outpatient clinic.

Can I take the test at home?

Pleural fluid testing cannot be conducted at home. The procedure to obtain pleural fluid must be performed by a trained health care professional in a medical setting like a doctor’s office or hospital.

How much does the test cost?

The cost of a pleural fluid test depends on several factors, including:

  • Whether you have health insurance
  • Whether your insurance covers the test
  • Where the thoracentesis procedure is conducted
  • What types of laboratory analysis are performed on the test sample

If you have health insurance, many of these costs are likely to be covered if the test is recommended by your doctor. However, you may still have costs toward a deductible or copayments depending on the specifics of your health insurance plan.

If you have questions or concerns about the cost of pleural fluid testing, talk with your doctor, an administrator at the facility where the test will be conducted, and/or a representative from your health insurance provider.

Taking a Pleural Fluid Test

The procedure to collect a sample from the pleural fluid is called thoracentesis, and it requires carefully inserting a needle into the pleural space around the lungs. This procedure is done by a health professional in a controlled medical environment like a hospital or doctor’s office.

In general, the needle is inserted through the back while a patient is sitting up. Less often, thoracentesis can be done while a person is lying on their back if they cannot sit up or if the pleural effusion cannot be reached from the back.

Before the test

There is normally no special preparation required for thoracentesis for a pleural fluid test. Before the procedure, you should tell your doctor about any medications that you take, especially any blood-thinning drugs.

During the test

During a thoracentesis procedure, you are usually sitting upright and may be asked to rest your arms on a table in front of you. Ultrasound is typically used to generate an image of the location of the pleural effusion near your lungs.

Once you are positioned, it is important that you do not move during the procedure. At points during the procedure, you may be asked to exhale or hold your breath.

While you are sitting up, the doctor identifies the specific area of your back where they will insert the needle. That area is thoroughly cleaned with an antiseptic. Then, a local anesthetic is applied to your skin in order to reduce pain.

A needle is inserted through your skin and muscle and into the pleural space around the lung. Fluid is then withdrawn through the needle. After enough fluid is collected, the needle is withdrawn, and a bandage is placed over the puncture site.

There may be some temporary pain or a sting when the needle is inserted. You may also feel the need to cough. It is important to tell the doctor if you feel any severe pain or shortness of breath while thoracentesis is being performed.

If you have a large pleural effusion that is causing symptoms, your doctor may put a thin tube called a catheter in place once the fluid for testing is removed. This allows for larger amounts of excess fluid to be drained over several hours or days.

After the test

After thoracentesis is complete, you may feel pressure in your chest, mild pain, or a need to cough. This feeling may last for a few hours as your lungs expand into the pleural space that no longer holds as much fluid. You may also feel light-headedness or dizziness, but these usually go away if you lie down for a few minutes.

Thoracentesis rarely causes serious side effects or complications. However, some potential risks may include:

  • Punctured lung: If the needle pierces the lung, it can cause air to build up in the pleural space and lead to a collapsed lung. This condition is also known as pneumothorax.
  • Bleeding: It is normal to have some bruising around the puncture site. However, an uncommon complication is bleeding from the procedure that causes the buildup of blood in or near the lung, requiring that blood to be drained.
  • Infection: The use of antiseptics greatly reduces the risk of infection, but it is possible for an infection to occur from the needle puncture.
  • Fluid buildup in the lung: Rarely, thoracentesis can lead to the accumulation of fluid in the lung, which is known as a pulmonary edema.
  • Punctured liver or spleen: This complication occurs if the needle pierces the liver or spleen during the procedure. Performing the thoracentesis with ultrasound and while you are sitting up makes this extremely unlikely.

An ultrasound is typically performed shortly after the procedure to check for potential complications. A chest x-ray may also be done in certain circumstances, including if the ultrasound is unclear or abnormal.

After the procedure, you should tell your doctor if you notice any breathing problems, signs of infection, or other lasting or bothersome side effects.

Pleural Fluid Test Results

Receiving test results

Results from a pleural fluid test will often be provided by your doctor either in person or over the phone. A test report showing the results may also be accessible electronically through an online health portal.

How long it takes to receive results can depend on the specific measurements and analyses performed on your sample. Many test results are available within a few business days, but others may take longer. Your doctor may want to see all of the test results before reviewing them with you.

Interpreting test results

Your health care provider plays a vital role in interpreting your pleural fluid test. To understand the test results, they will carefully consider and compare multiple test components.

For different measurements, the laboratory will show a reference range. This range explains what the laboratory expects that test level to be in a healthy person. These reference ranges can vary between laboratories, so it is necessary to look closely at your specific test report when reviewing results.

Pleural fluid test results are interpreted to understand the cause of a pleural effusion. Dozens of different conditions can cause a pleural effusion, and test results can be used to identify the most likely explanation.

One important step in this process is determining whether a pleural effusion is transudative or exudative:

  • A transudative pleural effusion involves watery fluid that has leaked into the pleural space. Transudative pleural effusions are most frequently caused by heart failure or liver or kidney problems.
  • An exudative pleural effusion involves protein-rich fluid entering the pleural space. There are more diverse potential causes of exudative pleural effusions including infections, inflammation, lung problems, tumors, and blockage of blood or lymph vessels.

Establishing whether a pleural effusion is transudative or exudative is typically based on levels of protein, LDH, and/or cholesterol in the pleural fluid.

Testing can also look for other underlying causes of a pleural effusion. For example, pleural fluid testing may look for evidence of conditions like:

  • Infection: If bacteria or fungi are found in the pleural fluid, it is often a sign of an infection. Pneumonia and tuberculosis are conditions caused by infections that may be detected through pleural fluid analysis.
  • Cancer: Testing may find cells that are cancerous, which can be the result of a tumor somewhere in the body. Types of cancer most often found in pleural fluid testing include breast cancer, lung cancer, lymphoma, stomach cancer, and ovarian cancer.
  • Empyema: Empyema is when pus enters the pleural space. Pus is a mixture of immune cells, dead tissue, and germs. Empyema frequently occurs as a result of an infection.
  • Hemothorax: Hemothroax is when blood is found in the pleural fluid. In most cases, this occurs because of an injury to the chest.
  • Chylothorax: Chylothorax is when lymphatic fluid is in the pleural space. This fluid is a component of the lymphatic system, which is part of the body’s immune defenses. Damage to the lymphatic system can cause chylothorax.
  • Urinothorax: Urinothorax is a rare condition that involves urine entering the pleural fluid. This can occur if the urinary system is blocked, preventing urine from moving properly from the kidneys and out of the body.

These are just some of the many possible findings from a pleural fluid test. Because so many conditions can cause pleural effusions, test results must be evaluated in the context of your symptoms, health history, and results from other tests.

Although a pleural fluid test is a useful diagnostic tool, this test alone cannot always identify the cause of a pleural effusion. Often other tests are needed, and in some cases, no specific cause can be detected.

If you are trying to understand the significance of your pleural fluid test, your doctor can review the specific findings from your test report and explain what they mean for you.

Are test results accurate?

A pleural fluid test is routinely performed in people who have a pleural effusion. Laboratories employ specific methods to analyze the fluid sample in order to reliably report test results. However, like any medical test, the accuracy of pleural fluid analysis may depend on the measurements included. If you have questions about the accuracy of your pleural fluid test, talk with the health care provider who ordered the test.

Do I need follow-up tests?

Follow-up tests may be necessary after an initial pleural fluid test. If the first test doesn’t demonstrate the cause of a pleural effusion, a repeat test with another thoracentesis may be needed.

On its own, pleural fluid testing can only diagnose a few specific causes of a pleural effusion. In many cases, other tests are required to establish a clear diagnosis. Examples of other tests that can be performed include:

  • Complete blood count (CBC)
  • Kidney function tests
  • Liver health tests
  • Urine protein tests
  • Ultrasound of the chest, heart, abdomen, or other areas
  • Computed tomography (CT) angiography of the chest, which is an imaging test that uses a dye to help visualize the blood vessels around the lungs
  • Bronchoscopy or thoracoscopy, which are procedures that involve putting a tube with a camera through the nose or mouth and into the lungs
  • Pleural biopsy, which uses a needle to remove a sample of tissue from the pleura that covers the lungs

Follow-up testing depends on your symptoms, overall health, and the results of your pleural fluid test. Talk to your doctor to understand the pros and cons of different kinds of follow-up tests.

Questions for your doctor about test results

Asking questions of your health care provider can empower you to be informed about your health. These questions may be helpful when reviewing your test results:

  • What are the findings from my pleural fluid test?
  • Based on the test results, what do you think is the most likely cause of the pleural effusion?
  • Do you recommend any follow-up testing?
  • Do you recommend any treatment or other steps to improve my health?


See More

Ask a Laboratory Scientist

Ask A Laboratory Scientist

This form enables patients to ask specific questions about lab tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. Please allow 2-3 business days for an email response from one of the volunteers on the Consumer Information Response Team.

Send Us Your Question