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Blood sample

Ammonia tolerance test for diagnosis of portosystemic shunts

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Blood sample
Image © iStockphoto.com/Dizzy

Normal hepatic function is essential for conversion of ammonia to urea. Increasing resting ammonia concentration indicates decreased hepatic mass or shunting of portal blood.

Concentrations of blood ammonia are not well correlated with severity of hepatic encephalopathy, and ammonia levels may be normal in 7% to 21% of dogs with portosystemic shunts (PSS), especially after prolonged fasting.

The ammonia tolerance test was developed to provide a more accurate diagnosis of liver dysfunction.

A heparinised baseline sample is taken after a 12 hour fast, and ammonium chloride is administered orally by stomach tube or in gelatin capsules or as an enema. A second blood sample is obtained 30 minutes after administration. Blood samples are then transported on ice for immediate plasma separation and analysis.

Normal values vary with the method of analysis; results in animals with PSS should be compared to a control sample from a healthy animal to ensure accuracy.

Comments

2 responses to “Ammonia tolerance test for diagnosis of portosystemic shunts”

  1. Ian Ramsey Avatar
    Ian Ramsey

    “”results in animals with PSS should be compared to a control sample from a healthy animal to ensure accuracy”
    This would suggest that you would give ammonium (sic) chloride to a healthy dog and then blood sample it. This would not be for the benefit of the healthy animal and therefore would not be a veterinary procedure. In the UK this would therefore likely to be under the animals scientific procedures act and appropriate licences might be required. In addition the risk of giving ammonium chloride to a dog with a portosystemic shunt would have to be considered carefully against the relative safety of ultrasonography, CT angiography or even mesenteric angiography to identify a shunting vessel.

    This is why this test is so rarely performed in the UK.

    Reply
  2. Vetsonline Avatar
    Vetsonline

    Thanks for your input, Ian – much appreciated. Thanks also for spotting the typo, which has now been amended.

    Reply

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