Peritoneal Fluid Analysis
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 comprises a group of tests used to determine the cause. There are two main reasons fluid may collect in the abdominal cavity:
- Fluid may accumulate because of 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 cirrhosis or congestive heart failure.
- Fluid accumulation may be caused by 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), rheumatioid disease, or systemic lupus erythematosus.
Differentiation between the types of fluid is important because it helps diagnose the specific disease or condition. Doctors and laboratorians use an initial set of tests (cell count, albumin 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 peritonitis and/or ascites.
How is the sample collected for testing?A sample of peritoneal fluid is collected by a doctor 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 peritoneal fluid accumulation (ascites) and/or inflammation of the peritoneum (peritonitis). There are two main reasons for fluid accumulation, and an initial set of tests (albumin, cell count and appearance) is used to differentiate between the two types of fluid that may be produced:
- 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 cirrhosis or congestive heart failure. If the fluid is determined to be a transudate, then usually no more tests on the fluid are necessary.
- Injury or inflammation of the peritoneum may cause abnormal collection of fluid (called an exudate). Exudates are associated with a variety of conditions and diseases, and several tests, in addition to the initial ones performed, may be used to help diagnose the specific condition including:
- Infectious diseases caused by viruses, bacteria, or fungi. Infections may originate in the peritoneum, be due to a rupture of the appendix, perforation of the intestines or the abdominal wall, contamination during surgery, or may spread to the peritoneum from other places in the body.
- Inflammatory conditions – peritonitis due to certain chemicals, irradiation, rarely due to an autoimmune disorder
- Malignancies – such as mesothelioma, tumor of the liver (hepatoma), lymphoma, or metastatic cancer.
- Other conditions –kidney disease and pancreatitis
When is it ordered?Peritoneal fluid analysis may be ordered when a doctor suspects that a patient has a condition or disease that is causing peritonitis and/or ascites. It may be ordered when a patient has:
- Ascites of unknown origin
- Abdominal pain and tenderness
- Intestinal perforation
- Suspected intra-abdominal malignancy
What does the test result mean?An initial set of tests performed on a sample of peritoneal fluid helps determine whether the fluid is a transudate or exudate.
- Physical characteristics—fluid appears clear
- Albumin level—decreased
- Cell count—few cells are counted
- Physical characteristics—fluid may appear cloudy
- 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 in the 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 order depending on the suspected cause:
Physical characteristics – the normal appearance of a sample of peritoneal fluid 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 the lymphatic system, and greenish from bile
- Reddish peritoneal fluid may indicate the presence of blood.
- Cloudy peritoneal fluid may indicate the presence of microorganisms and/or white blood cells pointing to an infection
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 pancreatic disease.
- Alkaline phosphatase—increased with damaged intestines
- 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 white blood cells (WBCs) but no red blood cells (RBCs) or microorganisms. Laboratories may examine drops of the peritoneal fluid and/or use a special centrifuge (cytocentrifuge) to concentrate the fluid’s cells at the bottom of a test tube. Samples are placed on a slide, treated with a special stain, and an evaluation of the different kinds of cells present is performed.
- 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 cytocentrifuged sample is treated with a special stain and examined under a microscope for abnormal cells and for white 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 microorganisms if infection is suspected.
- Gram stain – for direct observation of bacteria or fungi under a microscope. There should be no organisms present in peritoneal fluid.
- Bacterial culture and susceptibility testing—ordered to detect any microorganisms, which will grow in the culture. 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.
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 patient is positioned lying down with the head of the bed raised. A local anesthetic is applied and then the doctor 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.