Patient Test Information

Pleural Fluid Analysis

Formal name:

Pleural Fluid Analysis

Related tests:

Pericardial Fluid Analysis, Peritoneal Fluid Analysis, Gram Stain, Susceptibility Testing, Total Protein, Albumin, Glucose Tests, LD, CEA, Fungal Tests, AFB Testing, Adenosine Deaminase

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Why Get Tested?

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

When to Get Tested?

When a healthcare practitioner suspects that someone with chest pain, coughing, and/or difficulty breathing has a condition that is causing an accumulation of fluid in the chest cavity

Sample Required?

A volume of pleural fluid collected using a procedure called thoracentesis

Test Preparation Needed?


How is it used?

Pleural fluid analysis is used to help diagnose the cause of accumulation of fluid in the chest cavity (pleural effusion). There are two main reasons for fluid accumulation and an initial set of tests, including fluid protein, albumin, or LD level, cell count, and appearance, is used to differentiate between the two types of fluid that may be produced, transudate or exudate.

  • Transudate: an imbalance between the pressure within blood vessels (which drives fluid out of the blood vessel) and the amount of protein in blood (which keeps fluid in the blood vessel) can result in accumulation of fluid. Transudates are 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: injury or inflammation of the pleurae may cause abnormal collection of fluid. If the fluid is an exudate, then additional testing is often ordered. Exudates are associated with a variety of conditions and diseases, including:
    • Infectious diseases - caused by viruses, bacteria, or fungi. Infections may originate in the pleurae or spread there from other places in the body. For example, pleuritis and pleural effusion may occur along with or following pneumonia.
    • Bleeding - bleeding disorders, pulmonary embolism, or trauma can lead to blood in the 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 - idiopathic, cardiac bypass surgery, heart or lung transplantation, pancreatitis, or intra-abdominal abscesses

Additional testing on exudate fluid may include:

  • Pleural fluid glucose, lactate, amylase, triglyceride, and/or tumor markers
  • Microscopic examination - a laboratory professional may place a sample of the fluid on a slide and examine it under a microscope. Normal pleural fluid has small numbers of white blood cells (WBCs) but no red blood cells (RBCs) or microorganisms.
  • Cytology - a laboratory professional may use a special centrifuge (cytocentrifuge) to concentrate the 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 - for direct observation of bacteria or fungi under a microscope. There should be no organisms present in pleural fluid.
  • Bacterial culture and susceptibility testing - ordered to detect any bacteria that may be present in the 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 for infectious diseases, such as tests for viruses, mycobacteria (AFB testing), and parasites.

When is it ordered?

Pleural fluid analysis may be ordered when a healthcare practitioner suspects that a person has a condition or disease that is causing pleuritis and/or pleural effusion. It may be ordered when someone has 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 fluid accumulation. The initial set of tests performed on a sample of pleural fluid helps determine whether the fluid is a transudate or exudate:


Transudates are most often caused by either congestive heart failure or cirrhosis. Typical fluid analysis results include:

  • Physical characteristics–fluid appears clear
  • Protein, albumin, or LDH level–low
  • Cell count–few cells are present


Exudates can be caused by a variety of conditions and diseases. Initial test results may include:

  • Physical characteristics–fluid may appear cloudy
  • Protein, albumin, or LD level–high
  • Cell count–increased

Additional test results and their associated causes may include:

Physical characteristics - the normal appearance of a sample of pleural fluid is usually light yellow and clear. Abnormal results may give clues to the conditions or diseases present and may 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 that may be performed in addition to protein or albumin may include:

  • 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 leakage from the 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. Results of an evaluation of the different kinds of cells present may include:

  • Total cell counts–the WBCs and RBCs in the sample are counted. Increased WBCs may be seen with infections and other causes of pleuritis. Increased RBCs may suggest trauma, malignancy, or pulmonary infarction.
  • WBC differential–determination of percentages of different types of WBCs. 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 - these tests may be performed to look for microorganisms if infection is suspected:

  • Gram Stain–for direct observation of bacteria or fungi under a microscope. There should be no organisms present in pleural fluid.
  • Bacterial culture and Susceptibility Testing–If bacteria are present, susceptibility testing can be performed to guide 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 the infection will be identified in the report and susceptibility testing may be done to guide therapy.
  • Adenosine deaminase–a markedly elevated level in pleural fluid in a person with symptoms that suggest tuberculosis means it is likely that the person tested has a Mycobacterium tuberculosis infection in their pleurae. This is especially true when there is a high prevalence of tuberculosis in the geographic region where a person lives. (For more details, see the test article on Adenosine Deaminase.)

Other less common tests for infectious diseases may be performed and may identify a virus, mycobacteria (such as the mycobacterium that causes tuberculosis), or a parasite as the cause of an infection and fluid accumulation.

Is there anything else I should know?

A blood glucose, protein, Albumin, or LD may be ordered to compare concentrations with those in the pleural fluid.

What is being tested?

Thumbnail diagram of respiratory system

Pleural fluid is a liquid derived from the blood in the tiny blood vessels (capillaries) in the lungs. It is found in small quantities between the layers of the pleurae - membranes that cover the chest cavity and the outside of each lung. It serves as a lubricant for the movement of the lungs during breathing.

A variety of conditions and diseases can cause inflammation of the pleurae (pleuritis) and/or excessive accumulation of pleural fluid (pleural effusion). Pleural fluid analysis is a group of tests that evaluate this liquid to determine the cause of the increased fluid.

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

  • An imbalance between the pressure of the liquid within blood vessels, which drives fluid out of blood vessels, and the amount of protein in 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 the 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 the chest and may be seen in infections (pneumonia, tuberculosis), malignancies (lung cancer, metastatic cancer, lymphoma, mesothelioma), or other causes of inflammation (sarcoidosis, autoimmune diseases).

Determining the type of fluid present is important because it helps to shorten the list of possible causes of pleural effusion. Healthcare practitioners and laboratorians 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. If the fluid is an exudate, additional tests may be performed to further pinpoint the disease or condition causing pleuritis and/or pleural effusion. See the "The Test" tab for more on this.

How is the sample collected for testing?

A sample of pleural fluid is collected by a healthcare practitioner with a syringe and needle using a procedure called thoracentesis.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

  1. What is thoracentesis and how is it performed?

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

  2. Are there other reasons to do a thoracentesis?

    Yes. Sometimes it will be performed to drain excess pleural fluid - to relieve pressure on the lungs. A catheter tube may be used to drain larger amounts of fluid and to drain recurrent fluid accumulations.