Ascitic fluid

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 their 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:

• Transudate: due to the ultrafiltration of plasma (i.e. removal of fluid). It does not contain large proteins and only few cells.
• Exudate: due to leakage of whole contents of plasma (i.e. fluid, cells and proteins). Largely due to an inflammatory process.

In ascites, rather than using the terms transudate and exudate, we refer to raised portal pressure or normal portal pressure.

Raised portal pressure (transudate)

• Cirrhosis
• Acute liver failure
• Cardiac failure
• Constrictive pericarditis
• Budd-Chiari syndrome (hepatic venous thrombosis)
• Portal vein thrombosis
• Liver metastasis

Normal portal pressure (exudate)

• Pancreatitis
• Malignancy (peritoneal carcinomatosis - widespread tumour deposition)
• Infection (bacterial, fungal, tuberculosis)
• Nephrotic syndrome (typically low SAAG with low protein content)
• Protein-losing enteropathy
• Bowel obstruction
• Serositis (e.g. in connective tissue disease)

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:

• Exclusion of infection: known as spontaneous bacterial peritonitis (SBP)
• Determine aetiology of ascites: provisionally based on the serum ascites albumin gradient (SAAG)

This refers to the series of tests that should be requested on an ascitic fluid sample.

• Albumin: to determine the SAAG and risk of SBP
• White cell count (WCC): used to assess for SBP
• Microscopy, culture & sensitivity: additional samples can be sent in blood culture bottles
• Amylase: if pancreatitis suspected
• Triglycerides: can be raised in chylous ascites due to disruption of abdominal lymphatics
• Cytology: assess for malignant cells
• Lactate dehydrogenase (LDH): may be used to differentiate transudates (low <225 U/L) and exudates (high >225 U/L). SAAG preferred.

The serum ascites-albumin gradient (SAAG) is used to determine the cause of ascites.

The SAAG is used to determine whether the aetiology of ascites is from raised portal pressure (transudate) or normal portal pressure (exudate).

Formula

SAAG = serum albumin (g/dL or g/L) - ascitic fluid albumin (g/dL or g/L)

Analysis

• High SAAG (>1.1 g/dL or >11 g/L): transudate
• Low SAAG (<1.1 g/dL or < 11g/L): exudate

Example

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:

• Direct spread: bacterial translocation across the bowel wall
• Haematogenous spread: bacteria enter ascites via the bloodstream in the context of an immunosuppressed state

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%.

Interpretation

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

• SBP: >250/mm³ of WCC, predominantly neutrophils, cultures positive
• Culture negative SBP: >250/mm³ of WCC, predominantly neutrophils, cultures negative
• Bacterascites: <250/mm³ of WCC, culture-positive (suggests early-stage SBP or colonisation)
• Secondary bacterial peritonitis: multiple organisms on culture, seen in 5%. Suggestive of intra-abdominal pathology (e.g. perforation).

Treatment

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.

Last updated: November 2021