Pleural effusion is a buildup of fluid between the lung and pleura, caused by various diseases. Thoracentesis is crucial in diagnosis, with pleural fluid analysis providing information on the cause. Exudative effusions are often caused by infections or inflammation, while transudative effusions are caused by imbalances in chest cavity pressures. Other tests on the pleural fluid can aid in diagnosis.
A pleural effusion occurs when fluid collects in the area between the lung and the pleura, a membrane that sits between the lung and chest cavity. This fluid buildup can be caused by a number of different disease processes. Detection of a pleural effusion, by physical exam or radiography, requires investigation into the cause of the effusion. The most important part of making a differential diagnosis of pleural effusion is performing a thoracentesis and obtaining a sample of the pleural fluid. Analysis of this fluid provides a great deal of information about the cause.
Pleural effusions can be diagnosed based on the clinical history, physical examination, and radiographic findings. Patients might report symptoms such as shortness of breath or pain on deep inspiration. On physical examination, doctors may identify an area of reduced percussion resonance or a region of reduced breath sounds over the pleural effusion. Findings on chest x-ray may include blunting of costophrenic angles formed by the meeting of the ribs and diaphragm and areas of opacity within the lung fields.
After identifying its presence, the next step in the differential diagnosis of pleural effusion is to perform a procedure called thoracentesis. The importance of thoracentesis cannot be understated; in fact, doctors are taught to do it as soon as possible in the event of a pleural effusion. With this procedure, a sterile needle is inserted between the ribs to obtain a sample of the fluid. The procedure can be done with the aid of an ultrasound machine or it can be done using physical exam maneuvers to locate the effusion.
The pleural fluid obtained from thoracentesis is sent to the laboratory for a series of tests. The first step in diagnosis is to determine whether the fluid is an exudate or a transudate. Light criteria are traditionally used to differentiate exudates from transudates. Pleural effusions are considered exudates if the ratio of pleural fluid protein to serum protein concentration is greater than 0.5. Also, if pleural fluid lactate dehydrogenase (LDH) is greater than two-thirds of the upper limit of normal, or if the ratio of pleural fluid LDH to serum LDH is greater than 0.6, the pleural effusion is considered an exudate.
Knowing whether the pleural effusion is exudative or transudative is important for the diagnosis. Transudative pleural effusions are caused by imbalances in the pressures within the chest cavity. Examples of causes of transudative pleural effusion include congestive heart failure, nephrotic syndrome, and hypoalbuminemia. In contrast, exudative pleural effusions are more commonly caused by infectious or inflammatory states. Examples of causes of exudative pleural effusion include pneumonia, tuberculosis, cancer, and connective tissue disorders.
There are other ways that pleural fluid can be helpful in the differential diagnosis of pleural effusion. The fluid is often cultured to see if bacterial species can be grown. It can be sent for cytogenetic analysis to see if there is any evidence of malignancy. High levels of amylase in the fluid can suggest pancreatitis, esophageal rupture, or cancer. Very low glucose levels could indicate tuberculosis, lupus or rheumatoid arthritis.
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