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.