Ascites is the accumulation of ascitic fluid in the peritoneal cavity.
Many diseases can cause ascites, but the most common cause is portal hypertension, which is usually due to livercirrhosis.
Ascites does not typically become clinically detectable until there are at least 500mLs of fluid present.
If large amounts of fluid accumulate, the abdomen can become very distended and tense, causing the patient to feel short of breath (due to diaphragmatic splinting).
Analysis of ascitic fluid can help determine the underlyingcause and identify signsofinfection.
A sample of fluid is typically obtained using a needle and syringe (known as an “ascitic tap” or “paracentesis”) and sent for analysis.
Clinical features of ascites
Typical clinicalfeatures of ascites include:
Shortness of breath
The appearance of ascitic fluid
The appearance of ascitic fluid can be useful in narrowing the differential diagnosis. The table below summarises how the typical appearance of ascitic fluid varies depending on the underlying aetiology.
Ascitic fluid biochemistry
Biochemicalanalysis of ascitic fluid can provide useful insights which can help narrow the differential diagnosis. The table below summarises the typical patterns of biochemical findings which are associated with specific underlying disease processes.
Ascitic fluid microscopy
Ascitic fluid microscopy provides valuable information about the number and type of red and whitecells within the fluid which can help narrow the differential diagnosis.
Serum ascitic albumin gradient (SAAG)
The serum ascitic albumin gradient (SAAG) indirectly measures portal pressure and can be used to determine if ascites is due to portal hypertension.
SAAG =(serum albumin) – (ascitic fluid albumin)
A high SAAG (>1.1g/dL) suggests the ascitic fluid is a transudate.
A low SAAG (<1.1g/dL) suggests the ascitic fluid is an exudate.
Causes of a high SAAG
A highSAAG (i.e. transudate) suggests the presence of portalhypertension, which may be caused by:
Venous occlusion (e.g. Budd Chiari syndrome)
Fulminant hepatic failure
Causes of a low SAAG
Causes of a lowSAAG (i.e. exudate) include:
Another way of differentiating between an exudate and a transudate is to assess the ascitic fluid’s lactate dehydrogenase (LDH) level:
LDH <225 U/L = transudate
LDH > 225U/ L = exudate
This method has largely been replaced by the use of SAAG.