Pleural fluid analysis is used to help diagnose the cause of of the (pleuritis) and/or accumulation of fluid in the pleural space (pleural ). 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 ). Transudates are most often caused by 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 ). 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 , , or . 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 , or trauma can lead to blood in the pleural fluid.
- Inflammatory conditions – such as lung diseases, lung inflammation due to prolonged exposure to large amounts of asbestos (asbestosis), , or autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus.
- Cancer – such as lymphoma, , or cancer.
- Other conditions – , 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 utrasonography 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 or :
- 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 or congestive heart failure.
- 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:
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 and/or white blood cells.
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.
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 (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 or 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 or under a microscope. There should be no organisms present in pleural fluid.
- Bacterial 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 , mycobacteria (AFB smear and culture), and .