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Kansas State Veterinary Diagnostic Laboratory

Kansas State Veterinary Diagnostic Laboratory
Kansas State University
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Manhattan, KS 66506
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January 2020

Diagnostic Approach to Peritoneal Effusions

By Dr. Sasha Thomason

The presence of peritoneal effusion can be fairly easily diagnosed. However, discovering how or why it is present can be challenging. The quickest way to narrow a long list of differential diagnoses down to a few is to collect a sample of the fluid. Knowing whether you are dealing with a transudate, modified transudate (aka high protein transudate), or exudate will help guide your diagnostic approach.

Our Clinical Pathology service would be happy to provide a full peritoneal fluid analysis (CP-44) of the fluid you pull off the abdominal cavity. Before you send off the sample, you should do a quick total protein with a refractometer to determine the fluid type. Below is a table that outlines the general rule of thumb for determining fluid type:

If the protein is:The fluid type is a:
Less than 2.5 g/dLTransudate
2.5 – 3.5 g/dLModified transudate (aka high protein transudate)
Greater than 3.5 g/dL and usually cloudy or bloodyExudate

If the fluid is a transudate, the rule outs are portal hypertension or hypoalbuminemia due to liver failure, protein-losing enteropathy or protein-losing nephropathy. Performing a complete blood count, serum chemistry, and urinalysis is the next step. An abdominal ultrasound should be part of the workup as well. 

If you determine the fluid to be a modified transudate (aka high protein transudate), the most common rule out is right-sided heart failure. An echocardiogram would be the next appropriate diagnostic step before considering other rule outs.

If you determine the fluid to be an exudate, it is either due to a non-septic cause (bile, urine, neoplasia, etc.), septic cause (usually due to GI perforation) or hemorrhage. A diagnostic ultrasound is usually necessary to determine the diagnosis medically, although the fluid appearance will be helpful for diagnosis. A CBC and chemistry panel to assess overall health for anesthetic risk purposes should also be completed. Cases presenting with exudates are almost always emergency cases, and surgical intervention is often necessary.  

A guide to what appropriate specialty within the KSU-VHC when referring cases by fluid type is shown below. 

If you are dealing with a:Refer to a: 
TransudateInternal Medicine specialist
Modified transudateCardiologist
ExudateSurgeon
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