Analysis of ascitic fluid is important for the work-up of chronic liver disease.
Ascites refers to the presence of pathological fluid within the abdominal cavity. In men, no fluid should be present. In women, up to 20 mls may be considered normal depending on the timing of the menstrual cycle.
Ascites is most commonly associated with liver disease. In this context, ascites develops due to portal hypertension. This refers to increased pressure within the portal venous system that drains blood from the gastrointestinal tract to the liver. Other causes of ascites are broadly due to local infiltration (e.g. tumour) of the peritoneal lining, inflammation or infection.
Ascites can be broadly be divided into raised portal pressure (transudates) or normal portal pressure (exudate).
Ascites, like pleural fluid, can be broadly divided into transudates or exudates:
In ascites, rather than using the terms transudate and exudate, we refer to raised portal pressure or normal portal pressure.
An ascitic tap describes the routine medical procedure completed to obtain an ascitic fluid sample.
An ascitic TAP (abdominal paracentesis) is a routine medical procedure that can be completed with ultrasound guidance. It should be an aseptic non-touch procedure that can be completed at the bedside. It involves inserting a small green needle into the abdomen and taking a sample of ascitic fluid.
Ascitic fluid samples can be sent to the lab for analysis. The two main indications include:
This refers to the series of tests that should be requested on an ascitic fluid sample.
The serum ascites-albumin gradient (SAAG) is used to determine the cause of liver disease.
The SAAG is used to determine whether the aetiology of ascites is from raised portal pressure (transudate) or normal portal pressure (exudate).
SAAG = serum albumin (g/dL or g/L) - ascitic fluid albumin (g/dL or g/L)
A patient presents with new abdominal swelling and reduced mobility. The serum albumin level is 32 g/L. An ascitic TAP is performed using an aseptic non-touch technique. Samples are sent for albumin, white cell count, MC&S and cytology. The ascitic fluid albumin comes back as 9 g/L.
This gives a SAAG of 23 g/L (32 - 9 = 23).
Therefore, the aetiology of the ascites is due to raised portal pressure resulting in a transudate. This is most likely to be chronic liver disease or heart failure. Further investigations are utilised to determine the cause.
Spontaneous bacterial peritonitis (SBP) refers to bacterial infection in the ascitic fluid.
SBP is one of the more common complications associated with chronic liver disease. Patients with ascites are at risk of developing bacterial infections. When a bacterial infection occurs within ascites it is called SBP.
SBP is theorised to occur by two mechanisms:
SBP may present in a number of ways including peritonitis (i.e. abdominal pain), hepatic encephalopathy, upper gastrointestinal bleeding, acute kidney injury or overt sepsis. It complicates ~10% of inpatient admissions with decompensated chronic liver disease and requires early recognition and treatment
The one year survival following an episode of SBP is 30-50%.
SBP is defined as an ascitic fluid white cell count > 250/mm3 that is predominantly neutrophilic (i.e. the majority of white cells seen are neutrophils). Usually defined as >90%.
This is typically associated with a positive ascitic fluid culture. However, 60% of patients are culture-negative
Key definitions of SBP
Treatment is with broad-spectrum antibiotics (e.g. third-generation cephalosporin) unless sensitivities for a particular microorganism are available. They can be given on suspicion (i.e. before the ascitic TAP result) to prevent any delay in treatment. Following an episode of SBP, patients require prophylactic oral antibiotics to reduce the risk of further episodes. The typical choice is Rifaximin.
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