To help diagnose the cause of peritonitis, an inflammation of the membrane lining the abdomen, and/or peritoneal fluid accumulation, where fluid builds up in the abdomen or around internal organs (called ascites)
Peritoneal Fluid Analysis
When you have abdominal pain and swelling, nausea, and/or fever and your healthcare practitioner suspects you have peritonitis or ascites
A peritoneal fluid sample obtained by inserting a needle into the abdominal cavity
You will be asked to empty your bladder prior to sample collection.
Peritoneal fluid is a liquid that acts as a lubricant in the abdominal cavity. It is found in small quantities (generally 5-20 mL) between the layers of the peritoneum that line the abdominal wall. Peritoneal fluid acts to moisten the outside of the organs and to reduce the friction of organ movement during digestion and movement.
A variety of conditions and diseases can cause inflammation of the peritoneum (peritonitis) and/or excessive accumulation of peritoneal fluid (peritoneal effusion or ascites). Peritoneal fluid analysis is a group of tests that evaluate this liquid to determine the cause of the increased fluid.
The two main reasons that fluid may collect in the abdominal cavity are:
- An imbalance between the pressure within blood vessels—which drives fluid out of blood vessels—and the amount of protein in blood—which keeps fluid in blood vessels. The fluid that accumulates in this case is called a transudate. Transudates are most often caused by congestive heart failure, nephrotic syndrome, or hepatic cirrhosis.
- An injury or inflammation of the peritoneum, in which case the fluid is called an exudate. This type of fluid may be the result of conditions such as infection, malignancies (metastatic cancer, lymphoma, mesothelioma), pancreatitis, ruptured gallbladder, or autoimmune disease.
Determining the type of fluid present (transudate or exudates) is important because it helps narrow down the possible causes of fluid buildup or inflammation. Healthcare practitioners and laboratorians use an initial set of tests, including cell count, serum and fluid albumin levels, and appearance of the fluid, to distinguish between transudates and exudates. Once the fluid is determined to be one or the other, additional tests may be performed to further pinpoint the disease or condition causing ascites.
How is the sample collected for testing?
A sample of peritoneal fluid is collected by a healthcare practitioner with a syringe and needle using a procedure called paracentesis.
Is any test preparation needed to ensure the quality of the sample?
You will be asked to empty your bladder prior to sample collection.
How is it used?
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
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
When is it ordered?
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
What does the test result mean?
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:
- 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
- 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. (For more details, see the article on Adenosine Deaminase.)
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.
Is there anything else I should know?
What is paracentesis and how is it performed?
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 healthcare practitioner inserts the needle into the abdominal cavity and the sample is removed.
Are there other reasons to do a paracentesis?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.
Are any other procedures done to help evaluate conditions involving the peritoneum?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.
On This Site
Tests: Pleural Fluid Analysis, Pericardial Fluid Analysis, Gram Stain, Susceptibility Testing, Glucose Tests, Albumin, White Blood Cell Count, Red Blood Cell Count, AFB Testing, Lactate Dehydrogenase (LD), Adenosine Deaminase
Elsewhere On The Web
MedlinePlus Medical Encyclopedia: Peritonitis – spontaneous bacterial
MedlinePlus Medical Encyclopedia: Ascites
American Cancer Society: Malignant Mesothelioma Detailed Guide
National Cancer Institute: Malignant Mesothelioma