Formal Name
Pleural Fluid Analysis
This article was last reviewed on
This article waslast modified on 18 May 2020.
At a Glance
Why Get Tested?

To help diagnose the cause of inflammation of pleurae (pleuritis, pleurisy), accumulation of fluid in the pleural space (pleural effusion), or possible malignancy

When To Get Tested?

When a doctor suspects that someone with chest pain, coughing, and/or difficulty breathing has a condition associated with pleuritis and/or pleural effusion

Sample Required?

A volume of pleural fluid is collected by a doctor using a procedure called thoracentesis, in part to relieve pressure and for diagnostic purposes

Test Preparation Needed?


On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

Pleural fluid is found in the pleural cavity and serves as a lubricant for the movement of the lungs during inhalation and exhalation. It is derived from a plasma filtrate from blood capillaries and is found in small quantities between the layers of the pleurae – membranes that cover the chest cavity and the outside of each lung.

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 used to help find the cause of the problem. There are two main reasons why fluid may collect in the pleural space:

  • Fluid may accumulate in the pleural space because of an imbalance between the pressure 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 usually involves both lungs and is often a result of either cirrhosis of the liver or congestive heart failure.
  • Fluid accumulation may be caused by injury or inflammation of the pleurae, in which case the fluid is called an exudate. It usually involves one lung and may be seen in infections (pneumonia, tuberculosis, sarcoidosis), malignancies (lung cancer, metastatic cancer, lymphoma, mesothelioma), rheumatoid disease, or systemic lupus erythematosus.

Differentiation between the types of fluid is important because it helps diagnose the specific disease or condition. Doctors use an initial set of tests (cell count, albumin and appearance of the fluid) to distinguish between transudates and exudates. Once the fluid is determined to be one or the other, additional tests may be performed to further pinpoint the disease or condition causing pleuritis and/or pleural effusion.

Accordion Title
Common Questions
  • How is it used?

    Pleural fluid analysis is used to help diagnose the cause of inflammation of the pleura (pleuritis) and/or accumulation of fluid in the pleural space (pleural effusion). There are two main reasons for fluid accumulation, and an initial set of tests (albumin, cell count and appearance of the fluid) is used to differentiate between the two types of fluid that may be produced:

    • 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 (called a transudate). Transudates are most often caused by cirrhosis of the liver or congestive heart failure. If the fluid is determined to be a transudate, then usually no more tests on the fluid are necessary.
    • Injury or inflammation of the pleurae may cause abnormal collection of fluid (called an exudate). Exudates are associated with a variety of conditions and diseases and several tests, in addition to the initial ones performed, may be used to help diagnose the specific condition including:
  • When is it requested?

    Pleural fluid analysis may be requested when a doctor suspects that a patient has a condition or disease that is causing pleuritis and/or pleural effusion. It may be requested when a patient 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
    • Pleural effusion may not cause any symptoms
  • What does the test result mean?

    An initial set of tests performed on a sample of pleural fluid helps determine whether the fluid is a transudate or exudate.


    • Physical characteristics—fluid appears clear
    • Protein or albumin concentration—decreased
    • Cell count—few cells are counted

    Transudates usually require no further testing. They are most often caused by either cirrhosis or congestive heart failure.


    • Physical characteristics—fluid may appear cloudy
    • Protein or albumin concentration—higher than normal
    • Cell count—increased

    Exudates can be caused by a variety of conditions and diseases and usually require further testing to aid diagnosis. Exudates may be caused by, for example, infections, trauma, various cancers, or pancreatitis. The following is a list of additional tests that the doctor may request depending on the suspected cause:

    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:

    • Milky appearance may point to lymphatic system involvement
    • Reddish pleural fluid may indicate the presence of blood
    • Cloudy thick pleural fluid may indicate the presence of microorganisms and/or white blood cells

    Additional tests –tests that may be performed in addition to protein or albumin may include:

    Microscopic examination – Normal pleural fluid has small numbers of white blood cells (WBCs) but no red blood cells (RBCs) or microorganisms. Laboratories may examine the pleural fluid and/or 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 the different kinds of cells that may be present.

    • 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 lympocytes 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 tumour 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 is used to detect any microorganisms that may be present in the pleural fluid. If bacteria are present, susceptibility testing can be performed to guide antimicrobial therapy. If there are no microorganisms 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.
    • Adenosine Deaminase - rarely, this test may be used to help detect tuberculosis. Markedly elevated level in pleural fluid in a person with symptoms suggestive of tuberculosis means likely that person has Mycobacterium tuberculosis infection. Sensitivity of this tests in only about 80%.

    Other tests for infectious diseases that are less commonly used may include tests for viruses, mycobacteria (AFB smear and culture), and parasites.

  • Is there anything else I should know?

    A blood glucose, protein, or albumin result may be used to compare concentrations with those in the pleural fluid. In order to differentiate exudate from transudate the total protein and LDH are measured in both blood and pleural fluid.

  • 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 anaesthetic is applied and then the doctor inserts the needle into the pleural cavity and the sample is removed.

  • 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.