Peritoneal Fluid Analysis

Peritoneal fluid analysis is used to help diagnose the cause of fluid buildup in the abdomen (ascites) and/or inflammation of the peritoneum (peritonitis). There are two main reasons for fluid accumulation, and an initial set of tests is used to differentiate between the two types of fluid that may be produced, transudate or exudate. These tests typically include:

• Fluid albumin level—the serum-ascites albumin gradient (SAAG) calculation (serum albumin level minus the fluid albumin level) may be used to differentiate between transudates and exudates. A SAAG level of 1.1 g/dL or greater suggests the presence of a transudate and less than 1.1 g/dL, an exudate.
• Cell count and differential
• Appearance

Transudate—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 congestive heart failure , nephrotic syndrome, or hepatic cirrhosis. If the fluid is determined to be a transudate, then usually no more tests on the fluid are necessary.

Exudate—injury or inflammation of the peritoneum may cause abnormal collection of fluid (called an exudate). Exudates are associated with infections, malignancies, pancreatitis, ruptured gallbladder, or autoimmune disease. Laboratory tests may be performed to determine one of the following conditions:

• Infectious diseases caused by viruses, bacteria, or fungi; infections may originate in the peritoneum due to a rupture of the appendix, perforation of the intestines or the abdominal wall, contamination during surgery, or an infection from other places in the body that has spread to the peritoneum.
• Inflammatory conditions – peritonitis due to certain chemicals, irradiation, or rarely an autoimmune disorder
• Malignancies – such as mesothelioma, tumor of the liver (hepatoma), lymphoma, or metastatic cancer
• Pancreatitis – due to certain drugs, infections, blockages, and circulating lipids (fats)

Additional testing on exudate fluid may include:

• Peritoneal fluid glucose, amylase, tumor markers, bilirubin, creatinine, lactate dehydrogenase (LD)
• Microscopic examination – may be performed if infection or cancer is suspected; a laboratory professional may 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 abnormal cells, such as malignant cells (cancer cells).
• Gram stain – for direct observation of bacteria or fungi under a microscope
• Bacterial culture and susceptibility testing – ordered to detect the presence of any microbes that have grown in the culture and to guide antimicrobial therapy
• Less commonly ordered tests for infectious diseases, such as tests for viruses, mycobacteria (AFB testing in identifying tuberculosis), and parasites
• Adenosine deaminase – rarely ordered for detecting tuberculosis in peritoneal fluid

Peritoneal fluid analysis may be ordered when a healthcare practitioner suspects that a person has a condition or disease that is causing peritonitis and/or ascites. It may be ordered when someone has:

• Ascites of unknown origin
• Abdominal pain, bloating, and tenderness
• A hole or break in the intestinal wall (intestinal perforation)
• Suspected cancer of the abdominal lining

Test results can help distinguish between types of peritoneal fluid and help diagnose the cause of fluid accumulation. The initial set of tests performed on a sample of peritoneal fluid helps determine whether the fluid is a transudate or exudate. Findings may include:

Transudate

Most ascitic fluids are transudates and are caused by either congestive heart failure or hepatic cirrhosis. Typical fluid analysis results include:

• Physical characteristics—fluid generally appears clear or straw-colored
• Protein—less than 3 g/dL
• Albumin level—low (typically evaluated as the difference between serum albumin and peritoneal fluid albumin, termed serum-ascites albumin gradient, or SAAG; values above 1.1 g/dL are considered evidence of a transudate.)
• Lactate dehydrogenase (LD) fluid/serum ratio—less than 0.6
• Glucose—equal to glucose level in the blood
• Cell count—few cells are present, usually lymphocytes
• Specific gravity—less than 1.015

Exudate

• Physical characteristics—fluid may appear cloudy
• Protein—greater than 3 g/dL
• Albumin level—higher than in transudates (typically with a SAAG less than 1.1 g/dL)
• Lactate dehydrogenase (LD) fluid/serum ratio—greater than 0.6
• Glucose—less than 60 mg/dL
• Cell count—increased
• Specific gravity—greater than 1.015

Exudates can be caused by a variety of conditions and diseases and usually require further testing to aid in the diagnosis. Infections, trauma, various cancers, or pancreatitis may cause exudates. The following is a list of additional tests that a healthcare practitioner may order depending on the suspected cause and typical results.

Physical characteristics – the normal appearance of a peritoneal fluid sample is usually straw-colored and clear. Abnormal appearances may give clues to conditions or diseases present and may include:

• Yellow with liver disease, milky from obstruction of the lymphatic system, and greenish from bile
• Reddish peritoneal fluid may indicate the presence of blood, most often due to trauma.
• Cloudy peritoneal fluid may indicate the presence of microbes and/or white blood cells (WBCs), pointing to an infection. It may also indicate lymph system blockage or trauma.

Chemical tests – tests that may be performed in addition to albumin may include:

• Glucose—typically about the same as blood glucose levels; may be lower with infection
• Amylase—increased with pancreatitis
• Tumor markers—to identify type of malignancy

Microscopic examination – may be performed if infection or cancer is suspected; normal peritoneal fluid has small numbers of WBCs but no red blood cells (RBCs) or microbes. Results of an evaluation of the different kinds of cells present may include:

• Total cell counts—WBCs and RBCs in the sample are enumerated. Increased WBCs may be seen with infections and malignant conditions.
• WBC differential—determination of percentages of different types of WBCs; an increased number of neutrophils may be seen with bacterial infections.
• Cytology—a special centrifuge (cytocentrifuge) concentrates a small sample of cells that is then treated with a special stain to be examined under a microscope for abnormal cells and for white blood cell differentiation. The differential can help determine whether the cells are the result of an infection or the presence of a tumor.

Infectious disease tests – tests may be performed to look for microbes if an infection is suspected.

• Gram stain—for direct observation of bacteria or fungi under a microscope; there should be no microbes present in peritoneal fluid.
• Bacterial culture and susceptibility testing—if bacteria are present, susceptibility testing can be performed to guide antimicrobial therapy. If there are no microbes 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.
• Fungal tests—if a culture is positive, the fungus or fungi causing the infection will be identified in the report and susceptibility testing may be done to guide therapy.
• Adenosine deaminase—a markedly elevated level in peritoneal fluid in a person with symptoms that suggest tuberculosis means it is likely that person has a Mycobacterium tuberculosis infection in that region of the body. This is especially true when there is a high prevalence of tuberculosis in the geographic region where a person lives.

Other less common tests for infectious diseases may be performed to identify a virus, mycobacteria (such as the mycobacterium that causes tuberculosis), or a parasite as the cause of an infection and fluid accumulation.

A blood glucose or albumin may be ordered to compare concentrations with those in the peritoneal fluid. If a healthcare practitioner suspects that you have a systemic infection, then a blood culture may be ordered in addition to the peritoneal fluid analysis.

Paracentesis is the removal of peritoneal fluid from the abdominal cavity with a needle, tubing, and a container that may have a vacuum. The individual receiving that test is positioned lying down with the head of the bed raised. A local anesthetic is applied, then the health care practitioner inserts the needle into the abdominal cavity and the sample is removed.

Yes. Sometimes it will be performed to drain excess peritoneal fluid – to relieve pressure in the abdomen. The volume of fluid removed may be large – sometimes as much as four liters (1 gallon) or more. This may need to be repeated periodically with some diseases.

Yes. If abdominal bleeding is suspected but is not evident because there is no swelling of the abdomen, a peritoneal lavage may be performed. This is done by inserting a small tube (catheter) into the peritoneal space, infusing a small amount of sterile fluid, and then withdrawing the fluid to see if any blood is present. A cell count is usually performed on a sample of the fluid.