Ascitic Fluid Analysis

Introduction

This guide provides a structured approach to interpreting ascitic fluid results in an OSCE setting.

What is ascites?

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 liver cirrhosis.

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 underlying cause and identify signs of infection.

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 clinical features of ascites include:

• Abdominal distension
• Abdominal discomfort
• Weight gain
• Shortness of breath
• Reduced appetite

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

Biochemical analysis 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 white cells 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 calculation

SAAG = (serum albumin) – (ascitic fluid albumin)

Interpretation

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.

Lactate dehydrogenase

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This method has largely been replaced by the use of SAAG.

Editor

Andrew Gowland