There are various places in the body where fluids can accumulate. When fluids accumulate inside a cavity they are referred to as effusions. Effusions can occur in the pleural, pericardial, and peritoneal cavities of the body. Since it is not natural for fluids to accumulate in theses spaces it is important that they be tested by a laboratory in order to determine the cause of the accumulation. Effusions are classified as either transudates or exudates. Laboratory testing is performed in order to determine whether a fluid is a transudate or an exudate.
Classifying a fluid as a transudate or an exudate can help clinicians determine what disease process is causing fluid to accumulate and enables them to proceed with subsequent treatment. All organs in the body have their own linings that help to protect the organ and are filtered by the lymphatic system. In order for these linings to work properly they must be permeable and allow for the transfer of fluids, proteins, and other metabolites between them. A transudate is a fluid that accumulates in cavities due to a malfunction of the filtering membranes of cavity linings. A malfunction in a membrane can be caused by organ disease or by the back up of the lymphatic system. These systemic disease processes cause the fluid balance between the linings to become disrupted which in turn leads to the buildup of fluid on one side of the membrane. Transudates are usually found in conditions such as liver disease, pancreatic disease, and congestive heart failure. An exudate is a fluid that accumulates inside a cavity due to the presence of foreign materials such as bacteria, viruses, parasites, fungi, and tumor cells. The presence of infection or cancerous cells causes a response by the body that sends large numbers of leukocytes to respond to the site. An exudate forms as a result of all these cells (both leukocytes and foreign material) and their metabolites filling the cavity.
Several tests can be performed in the laboratory to determine if a fluid is either a transudate or an exudate. The results of these tests, specifically the chemistries, should be compared with baseline peripheral blood testing in order to determine the whether the results obtained from the fluid sample are normal or abnormal. This is done at the discretion of the clinician as most laboratories do not provide reference ranges for fluids. The major test used to differentiate between a transudate or an exudate is the concentration of total protein in a fluid. Transudates generally have total protein concentration less than 3.0 g/dL while exudates generally have a total protein greater than 3.0 g/dL. Another way of looking at this value is to compare it to the level of total protein found in the patient’s serum. A transudate will still have a low concentration of total protein while an exudate will generally have a concentration of total protein that is greater than half the concentration of total protein found in the serum. Other chemistry testing that can help to differentiate transudates from exudates includes lactate dehydrogenase (LDH), glucose, and amylase. Lactate dehydrogenase is an enzyme that is used by cells in metabolism and production of energy. When there is a large presence of cells and cell death such as in infection and inflammation the concentration of LDH in the area increases. Transudates will have LDH levels lower than 200 units/L while exudates will have LDH levels higher than 200 units/L. Another way of assessing the levels of LDH in fluid is to compare it to the concentration of LDH in serum. Transudates will have a fluid to serum ratio of LDH lower than 0.6 while exudates will have a ratio of fluid to serum LDH that is higher than 0.6. While the concentration of glucose in a fluid does not necessarily help to determine if a fluid is a transudate or an exudate, it can help determine what might be causing an exudate. Decreased values in the concentration of glucose in an exudate can occur in bacterial infections, malignancies, rheumatoid arthritis, and tuberculosis. The glucose levels in an exudate will be considerably lower in comparison with the serum glucose in these conditions. Like glucose, the purpose for measuring the concentration of amylase in a fluid is to help determine what might be causing an exudate. Concentrations of amylase can accumulate in an exudate in response to esophageal rupture, pancreatitis, and pancreatic cancer. No matter what chemistry testing is ordered on a fluid it is best to compare it in relation to serum levels to help assess whether a fluid is a transudate or an exudate.
Other testing that can help to differentiate between a transudate and an exudate includes noting the appearance of the fluid and performing a differentiated cell count. Transudates are generally clear and pale yellow in appearance as they are basically filtrates of plasma. These fluids contain very little cellular material. The leukocyte count is usually less than 1.0 X 109/L and the erythrocyte count is less than 100.0 X 109/L. The leukocytes that are present consist of monocytes and lymphocytes. On the other hand exudates have an opposite appearance. Exudates will generally appear cloudy or turbid. They can have a variety of colors; yellow, brown, greenish, and even bloody. In some instances they may even be clotted due to the presence of fibrinogen. Exudates will show an abundance of cellular material. The leukocyte count will usually be greater than 1.0 X 109/L and include neutrophils, lymphocytes, monocytes, eosinophils, and even basophils. The erythrocyte count will generally be greater than 100.0 X 109/L, however the count can be falsely increased in the event of a traumatic tap. In instances that an exudate is caused by infection it might be possible to visualize bacteria with a Wright’s stain; however it is more useful to perform a Gram Stain. Finally a cell count and differential can reveal the presence of cancer. Tumor cells can be seen in a peripheral smear and be sent for pathology review in order to determine what type of cancer is present in the patient. The results from both the hematology and chemistry testing of a fluid can help a clinician to determine what type of fluid is being produced by the body and why it is being produced.
Effusions that form within the body are classified as either transudates or exudates. Buildup of fluid most often occurs in the pleural, pericardial, and peritoneal cavities of the body. While it is natural to have a balance of fluid between these linings for the purpose of protection and lymphatic drainage, disease processes can upset this balance. Laboratory testing is performed in order to determine the cause for this buildup of fluid. Classifying a fluid as a transudate or and exudate allows clinicians to determine what disease processes might be occurring and how to subsequently treat them.
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