Blood-tests Explained


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This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Pleural Fluid Analysis


At a Glance

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?

None

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.

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

No test preparation is needed.

The Test