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 comprises a group of tests used to determine the cause. There are two main reasons 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 or congestive heart failure.
• Fluid accumulation may also be caused by injury or inflammation of the pleurae, in which case the fluid is called an exudate. It usually involves only 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 and laboratory scientists 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.

Pleural fluid analysis is used to help diagnose the cause of inflammation of the pleurae (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 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:

• 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 due to prolonged exposure to large amounts of asbestos (asbestosis), sarcoidosis, or autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus.
• Cancer – such as lymphoma, mesothelioma, or metastatic cancer.
• Other conditions – idiopathic, cardiac bypass surgery, heart or lung transplantation, or pancreatitis.

Pleural fluid analysis is requested after the information from a detailed history and physical examination, review of blood tests, chest imaging by X-ray and/or ultrasonography have been evaluated by the doctor.

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

Transudate

• Physical characteristics - fluid appears clear
• Protein or albumin level - low compared to serum or plasma total protein or albumin
• Lactate dehydrogenase(LD) - low compared to serum or plasma LD
• Transudates usually require no further testing. They are most often caused by either cirrhosis or congestive heart failure.
   

Exudate

• Physical characteristics - fluid may appear cloudy
• Protein or albumin level - similar to the plasma total protein or albumin
• Lactate dehydrogenase (LD) - similar to or higher than the plasma LD result
• Exudates can be caused by a variety of conditions and diseases and usually require further testing to aid in the 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 order 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 and may include:

• Milky appearance may point to thoracic duct involvement (see below).
• Reddish pleural fluid may indicate the presence of blood.
• Cloudy thick pleural fluid may indicate the presence of microorganisms and/or white blood cells.

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, malignancy and rheumatoid arthritis.
• Amylase levels may increase with pancreatitis, oesophageal rupture, or malignancy.
• Triglyceride levels may be increased with thoracic duct involvement. The thoracic duct is the biggest lymph duct in the body.
• Tumour markers 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. 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 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 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 ordered 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.
  Other tests for infectious diseases that are less commonly ordered may include tests for viruses, mycobacteria (AFB smear and culture), and parasites.

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

RCPA Manual: Pleural/pericardial fluid examination

Healthdirect Australia: Pleurisy