Peritoneal fluid analysis

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, tumour of the liver (hepatoma), lymphoma, or metastatic cancer.
  • Other conditions - kidney disease and pancreatitis

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

An initial set of tests performed on a sample of peritoneal fluid helps determine whether the fluid is a transudate or exudate.

Transudate:

• Physical characteristics - fluid appears clear
• Albumin level - decreased
• Cell count - few cells are counted

Transudates usually require no further testing. Ninety percent of ascitic fluids are transudates and are caused by either cirrhosis or congestive heart failure.

Exudate:

• 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-coloured 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
• Tumour 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 tumour.

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.

Other peritoneal fluid tests for infectious diseases that are less commonly ordered include tests for viruses, mycobacteria (AFB smear and culture) or parasites.

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