Pleural Fluid Analysis

• Physical characteristics—fluid appears clear
• Protein or albumin level—decreased
• Cell count—few cells are counted

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—higher than normal
• Cell count—increased

Exudates can be caused by a variety of conditions and diseases and usually require further testing to aid 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 request 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:

• Milky appearance may point to lymphatic system involvement
• 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 and rheumatoid arthritis.
• Lactate levels can increase with infectious pleuritis, either bacterial or tuberculosis.
• Amylase levels may increase with pancreatitis, oesophageal rupture, or malignancy.
• Triglyceride levels may be increased with lymphatic system involvement.
• 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 lympocytes 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 tumor 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 used 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 used may include tests for viruses, mycobacteria (AFB smear and culture), and parasites.