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Pleural Fluid Analysis

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Formal name: Pleural Fluid Analysis

The Test Sample

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 in the lungs 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 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 usually involves both lungs 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 usually involves one lung and may be seen in infections (pneumonia, tuberculosis, sarcoidosis), malignancies (lung cancer, metastatic cancer, lymphoma, mesothelioma), or autoimmune disease.

Differentiation between the types of fluid is important because it helps diagnose the specific disease or condition. Doctors and laboratorians use an initial set of tests (cell count, protein, albumin, or lactate dehydrogenase (LDH) level, 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.

How is the sample collected for testing?

A sample of pleural fluid is collected by a doctor 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.