What is Ascitic / Peritoneal Fluid Analysis?

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. The 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 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
• Appearance

Transudate—an imbalance between the pressure within blood vessels (which drives fluid out of the blood vessel) and the amount of protein in the 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 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