Postprandial venous ammonia concentrations in the diagnosis of hepatobiliary disease in dogs

A postprandial ammonia tolerance test (PPATT) was performed on normal dogs and dogs with signs that suggested they may have liver disease. All dogs un derwent transcolonic scintigraphy, liver biopsy, or both and were assigned to extrahepatic disease, primary hepatocellular, and congenital portosystem ic vascular anomalies (PSVA) groups. Each dog was fed a chicken and rice di et providing 25% of its estimated daily metabolizable energy requirement (M ER) as an ammonia challenge. This is practical in patients with liver disea se because ammonium chloride administration often causes vomiting or ammoni a toxicity. Venous ammonia concentrations were measured before feeding and every 2 hours after feeding for 8 hours. No difference in mean ammonia conc entrations between dogs with extrahepatic disease and control dogs was foun d. Therefore, the specificity of the PPATT was 100%. Dogs with hepatocellul ar disease showed no change in mean ammonia concentration at any time point , before or after feeding, but sensitivity was greatest when venous ammonia was measured 6 hours after feeding (sensitivity before feeding, 28%, and a fter feeding, 36%). Among dogs with congenital PSVA, mean ammonia concentra tions were higher than the reference range at all time points before and af ter feeding, and peak mean ammonia concentration occurred 6 hours after fee ding. In this group, the sensitivity of the PPATT was 81% before feeding an d 91% 6 hours after feeding. This study demonstrates that the measurement o f venous ammonia concentration is a useful test to detect congenital PSVA. and the sensitivity of the test may be improved by sampling 6 hours after f eeding. The PPATT has poor sensitivity in detecting primary hepatocellular disease.