Peritoneal Fluid Analysis
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
A peritoneal fluid analysis is a group of tests that examine fluid collected from the abdominal cavity using a sample obtained by a procedure called paracentesis. It is used to help diagnose the cause of fluid build-up (ascites) and to investigate conditions such as infection, inflammation, liver disease or cancer affecting the abdomen.
Why get tested?
To help diagnose the cause of peritonitis and/or peritoneal fluid accumulation (ascites)
When to get tested?
When a doctor suspects that someone with abdominal pain and swelling, nausea, and/or fever has a condition associated with inflammation of the peritoneum (peritonitis) or peritoneal fluid accumulation
Sample required?
A peritoneal fluid sample obtained by inserting a needle into the abdominal cavity
Test preparation needed?
You will be asked to empty your bladder prior to sample collection.
What is being tested?
Peritoneal fluid is a liquid that acts as a lubricant in the abdominal cavity. It is found in small quantities between the layers of the peritoneum. Peritoneal fluid is produced by mesothelial cells in the membranes and acts to moisten the outside of the organs and to reduce the friction of organ movement during digestion.
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 help find the cause of the problem. There are two main reasons why 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, cancer (metastatic cancer, lymphoma, mesothelioma), rheumatoid disease, or systemic lupus erythematosus.
Differentiation between the types of fluid is important because it helps diagnose the specific disease or condition. Doctors use an initial set of tests (cell count, albumin or protein and appearance of the fluid) to distinguish between transudates and exudates. Although the total protein content has been used as the major criterion for many years to differentiate transudate and exudate causes of ascites, the concentration of protein in the fluid needs to be compared to the concentration in the blood. This can be done by measuring the albumin in the blood and calculating the serum ascites-albumin gradient (SAAG).
Additional tests may be performed to identify the disease or condition causing peritonitis and/or ascites. Ascitic fluid amylase, fluid culture and cytology are some of common additional tests.
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.
Common questions
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 (protein or albumin, cell count and appearance) is used to differentiate between the two types of fluid that may be produced (exudate and transudate).
Peritoneal fluid analysis may be requested when a doctor suspects that a patient has a condition or disease that is causing peritonitis and/or ascites. It may be used 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. This differentiation is mainly based on protein content of the fluid. Protein concentration is high in exudative ascites and low in transudative ascites but, sometime there is an overlap. The gradient between blood albumin concentration and ascitic fluid albumin concentration (SAAG) has 97% accuracy.
The gross appearance of the fluid can provide useful diagnostic information. In cirrhosis, the fluid usually has a clear straw-coloured appearance. A blood-stained sample is usually due to a traumatic tap and in these cases the fluid tends to clot on standing. Samples which remain homogeneously blood stained throughout the tap could indicate malignancy, pancreatitis, TB, intestinal infarction or recent abdominal trauma.
Ascitic fluid amylase is diagnostic of ascites secondary to pancreatic disease. Therefore, this is usually performed in patients with suspected pancreatic disease. High neutrophil count (type of white blood cells) >250 cells/mm³ is diagnostic of spontaneous bacterial peritonitis. The concentration of red blood cells is usually lower than 1000 cells/mm³ in ascitic fluid.
Cytology examination of ascitic fluid can help diagnose malignant causes of ascites. Gram staining of ascitic fluid is usually not indicated as it is very rarely helpful. Ascitic fluid culture for mycobacteria has about 50% sensitivity of diagnosis.
To help identification of organism in infections, it is useful to culture ascitic fluid in blood culture inoculation bottles.
Other tests: Lactate dehydrogenase activity and tumour markers are tested in certain malignancies causing ascites. Adenosine deaminase activity is checked in ascites caused by tuberculosis.
Glucose level in ascitic fluid usually reflects prevailing blood levels unless it is used by bacteria and white blood cells in ascitic fluid. Therefore, decreased glucose levels may support tuberculous or bacterial peritonitis and ascites due to malignancies.
If a doctor suspects that a patient may have a systemic infection, then a blood culture may be requested in addition to the peritoneal fluid analysis.
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 anaesthetic is applied and then the doctor inserts the needle into the abdominal cavity and the sample is removed.
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 litres (1 gallon) or more. This may need to be repeated periodically with some diseases. Laboratory testing of fluid in this situation is not required.
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. This is rarely performed now with the availability of CT scanning.