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 (fluid albumin level, cell count and differential, 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 congestive heart failure or cirrhosis. 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
Microscopic examination – may be performed if infection or cancer is suspected. 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.
Gram stain – for direct observation of bacteria or fungi under a microscope
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:
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.)
Cell count—few cells are present
Physical characteristics—fluid may appear cloudy
Albumin level—higher than in transudates (typically with a SAAG less than 1.1 g/dL)
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 and typical results.
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:
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. 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 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—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.
Less commonly, if testing for other infectious diseases is performed and is positive, then the cause of the peritoneal fluid accumulation may be due to a viral infection, mycobacteria (such as the mycobacterium that causes tuberculosis), or a parasite.
A blood glucose or albumin may be ordered to compare concentrations with those in the peritoneal fluid. If a doctor suspects that a person may have a systemic infection, then a blood culture may be ordered in addition to the peritoneal fluid analysis.
This article was last reviewed on July 8, 2012. | This article was last modified on October 8, 2014.
The review date indicates when the article was last reviewed from beginning to end to ensure that it reflects the most current science. A review may not require any modifications to the article, so the two dates may not always agree.
The modified date indicates that one or more changes were made to the article. Such changes may or may not result from a full review of the article, so the two dates may not always agree.