Pleural Fluid Testing

This article was last modified on August 11, 2020

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Pericardial Fluid Analysis

At a Glance

Why Get Tested?

To help diagnose the cause of accumulation of fluid in your chest cavity (pleural effusion)

When To Get Tested?

When a healthcare practitioner suspects that you have an accumulation of fluid in your chest cavity because of chest pain, coughing, and/or difficulty breathing; and after being confirmed by a chest x-ray

Sample Required?

A volume of pleural fluid collected using a procedure called thoracentesis (see Common Questions)

Test Preparation Needed?

None

What is being tested?

The exterior surface of your lungs and thoracic cavity are lined with continuous membrane known as the pleura. Pleural fluid is layered between the two surfaces so that the opposing surfaces with “glide” across each other during breathing. This “gliding” movement helps with normal breathing. In healthy individuals, there is about 10 to 20 milliliters of pleural fluid that is evenly distributed across the pleura and it is continually replenished from the blood in the tiny blood vessels (capillaries) in your lungs. Certain diseases will increase the pleural volume, usually as a pocket or collection. In these cases, it should be tested and, if necessary, drained.

A variety of conditions and diseases can cause inflammation of these membranes (pleuritis) and/or excessive buildup of pleural fluid (pleural effusion). Pleural fluid testing evaluates this liquid to determine the cause of the increased fluid.

The two main reasons for fluid buildup in the pleural space are:

An imbalance between the pressure of the liquid within your blood vessels, which drives fluid out of blood vessels, and the amount of protein in your blood, which keeps fluid in blood vessels. The fluid that accumulates in this case is called a transudate. This type of fluid more commonly involves both sides of your chest and is most frequently a result of either congestive heart failure or cirrhosis.
An injury to or inflammation of the pleurae, in which case the fluid that accumulates is called an exudate. It more commonly involves one side of your chest. Exudates are associated with a variety of conditions and diseases, including:
- Infections—caused by viruses, bacteria, or fungi. Infections may start in the pleurae or spread there from other places in your body. For example, pleuritis and pleural effusion may occur along with or following pneumonia.
- Bleeding—bleeding disorders, trauma, or blockage in a lung artery (pulmonary embolism) can lead to blood in your pleural fluid.
- Inflammatory conditions—such as lung diseases, chronic lung inflammation for example due to prolonged exposure to large amounts of asbestos (asbestosis), sarcoidosis, or autoimmune disorders such as rheumatoid arthritis and lupus
- Malignancies—such as lymphoma, leukemias, lung cancer, metastatic cancers
- Other conditions—examples include unknown (idiopathic), heart bypass surgery, heart or lung transplantation, pancreatitis, or abscesses within the abdomen

Determining the type of fluid present is important because it helps to shorten the list of possible causes of your pleural effusion. Healthcare practitioners and laboratory professionals use an initial set of tests (cell count, protein, albumin, and lactate dehydrogenase (LD) level, and appearance of the fluid) to distinguish between transudates and exudates; part of this evaluation is collection of a blood specimen to compare levels between blood and effusion. If the fluid is an exudate, additional tests may be performed to further pinpoint the disease or condition causing your pleuritis and/or pleural effusion.

Common Questions

How is the test used?

Pleural fluid testing is used to help diagnose the cause of fluid buildup in your chest cavity (pleural effusion). An initial set of tests typically includes:

Fluid protein, albumin, or LD level
Cell count
Fluid appearance

Results from these are compared to results from a blood specimen to determine whether the fluid is a transudate or an exudate.

Transudate—this is most frequently caused by congestive heart failure or cirrhosis. If the fluid is determined to be a transudate, then usually no more tests on the fluid are necessary.
Exudate—additional testing is often ordered to diagnose the cause and may include:
- Tests to measure pleural fluid glucose, lactate, amylase, triglyceride levels and tests for tumor markers such as CEA may be done, though the usefulness of these tests is not clear.
- Microscopic examination – a laboratory professional places a sample of your fluid on a slide and examines it using a microscope, counting any white blood cells (WBCs) and red blood cells (RBCs) and looking for bacteria or fungi.
- Cytology – a laboratory professional may use a special centrifuge (cytocentrifuge) to concentrate your fluid’s cells on a slide. The slide is treated with a special stain and evaluated for abnormal cells, such as malignant cells (cancer cells).
- Gram stain – used to look for bacteria or fungi using a microscope; there should be no organisms present in your pleural fluid.
- Bacterial culture and susceptibility testing – used to identify any bacteria that may be present in your pleural fluid and to guide antimicrobial therapy
- Fungal tests – may include fungal culture and susceptibility testing
- Adenosine deaminase – may help detect tuberculosis (TB)
- Less commonly, tests may be used to identify infections caused by viruses, mycobacteria (AFB testing), and parasites.

When is it ordered?

Pleural fluid testing may be ordered when your healthcare practitioner suspects that you have pleuritis and/or pleural effusion after confirmation with a chest x-ray. It may be ordered when you have some combination of the following signs and symptoms:

Chest pain that worsens with deep breathing
Coughing
Difficulty breathing, shortness of breath
Fever, chills
Fatigue

What does the test result mean?

Test results can help distinguish between types of pleural fluid and help diagnose the cause of your fluid accumulation. The initial set of tests performed on a sample of your pleural fluid helps determine whether the fluid is a transudate or exudate:

Fluid Appearance	Protein, Albumin or LD Level	Cell Count	Type of Fluid	Cause
Clear	Low	Few cells	Transudate	CHF or cirrhosis
May be cloudy	High	Increased	Exudate	Several possible causes, additional tests needed

If the fluid is an exudate, additional test results and their associated causes may include:

Fluid appearance – pleural fluid is usually light yellow and clear. Abnormal results may give clues to the conditions or diseases present. Examples include:

Reddish pleural fluid may indicate the presence of blood.
Cloudy, thick pleural fluid may indicate an infection and/or the presence of white blood cells. It may also indicate leakage of fluid from the lymphatic system (lymph). Lymph drains from the lymphatic system into the venous system in the chest and either trauma or lymphoma can cause lymph to be present in pleural fluid.

Chemical tests – tests performed in addition to protein or albumin may include the following, although the evidence on their usefulness is not clear:

Glucose—typically about the same as blood glucose levels; may be lower with infection and rheumatoid arthritis.
Lactate levels can increase with infections.
Amylase levels may increase with pancreatitis, esophageal rupture, or malignancy.
Triglyceride levels may be increased when there is a leak from your lymphatic system.
Tumor markers, such as CEA, may be increased with some cancers.

Microscopic examination – normal pleural fluid has small numbers of white blood cells (WBCs) but no red blood cells (RBCs) or microorganisms.

Total cell counts—increased WBCs may be seen with infections and other causes of pleuritis. Increased RBCs may suggest trauma, malignancy, or pulmonary infarction.
WBC differential—identifies different types of WBCs present; an increased number of neutrophils may be seen with bacterial infections. An increased number of lymphocytes may be seen with cancers and tuberculosis.
Cytology—a cytocentrifuged sample is treated with a special stain and examined under a microscope for abnormal cells. This is often done when a mesothelioma or metastatic cancer is suspected. The presence of certain abnormal cells, such as tumor cells or immature blood cells, can indicate what type of cancer is involved.

Infectious disease tests:

Gram stain—there should be no organisms present in your pleural fluid.
Bacterial culture and susceptibility testing—if bacteria are present, susceptibility testing can be performed to guide your antimicrobial therapy. If there are no bacteria present, it does not rule out an infection; they may be present in small numbers or their growth may be inhibited because of prior antibiotic therapy.
Fungal tests—if a culture is positive, the fungus or fungi causing your infection will be identified in the report and susceptibility testing may be done to guide therapy.
Adenosine deaminase—a markedly elevated level in your pleural fluid when you have symptoms that suggest tuberculosis means it is likely that you have a Mycobacterium tuberculosis infection. This is especially true if you live in an area where tuberculosis is common. (For more details, see the test article on Adenosine Deaminase.)

Other less common tests for infections may be performed and may identify, for example, a virus, the mycobacterium that causes tuberculosis, or a parasite as the cause.

What is thoracentesis and how is it performed?

Thoracentesis is the removal of pleural fluid from your pleural cavity with a needle and syringe. You are positioned sitting upright with arms raised and supported. A local anesthetic is applied and then the healthcare practitioner inserts the needle into your chest (pleural) cavity and the sample is removed.

Are there other reasons to do a thoracentesis?

Yes. Sometimes it will be performed when you have a pleural effusion to drain the excess fluid and to relieve pressure on your lungs. A catheter tube may be used to drain larger amounts of fluid and to drain fluid that continues to accumulate.

Can a pleural effusion go away on its own?

Yes, if a small amount of fluid has built up, then it may go away without treatment.

Is a pleural effusion serious?

A pleural effusion can be serious if the excess fluid puts pressure on your lungs, making it difficult to breathe. Pleural effusions are not normal and must be evaluated to determine the cause. Also, the cause of the pleural effusion can be serious and require extensive treatment. Serious causes can include heart failure, pneumonia and other infections, cancer, a blocked lung artery (pulmonary embolism) and liver disease.

Is there anything else I should know?

A blood glucose, protein, albumin, or LD test may be ordered to compare levels with those in your pleural fluid.

Related Images

Diagram of the Respiratory System, including the Lungs. Image credit: National Cancer Institute

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