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Pleural Fluid Analysis

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Formal name: Pleural Fluid Analysis

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The Test Sample

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.