QUARTERLY BEAT / OCTOBER 2025
HIGH ALT & ALP IN DOGS:
OR DO ALL THE TESTS?
IGNORE,
What should you do when your canine patient’s ALT or ALP is elevated? In this VETgirl article, Dr. John Sessions, DACVIM (SAIM), reviews how to interpret liver enzyme changes and when to pursue further diagnostics. Read on for practical strategies to better diagnose and manage common hepatobiliary diseases in dogs.
John Sessions, DVM, DACVIM Nashville Veterinary Specialists, Nashville, TN
INTRODUCTION In this session, we delve into the clinical evaluation of hepatobiliary diseases, particularly focusing on diagnostic methods, tests, and clinical management strategies associated with elevated liver enzymes in dogs. Liver diseases are commonly encountered in veterinary practice, and an understanding of the various diagnostic tools and disease-specific considerations is vital for accurate diagnosis and effective treatment.
TESTS FOR CHOLESTASIS AND DRUG INDUCTION In addition to ALT and AST, alkaline phosphatase (ALP) and gamma- glutamyltransferase (GGT) are commonly measured to evaluate cholestasis or biliary tract disease. ALP is typically not present in normal hepatic tissue but can be induced by obstructed bile flow or certain drugs, including glucocorticoids and anticonvulsants. GGT is more specific for liver disease, with elevated levels strongly indicating hepatobiliary disorders, such as cholangiohepatitis or bile duct obstruction. When both ALP and GGT are elevated, the likelihood of liver disease is increased to 94%. EVALUATION OF LIVER FUNCTION Liver function is not solely determined by enzyme activity. Several other biochemical tests, including albumin, blood glucose, bilirubin, and major clotting factors, provide insights into liver function. Particularly notable is the role of bile acids in assessing hepatobiliary function. Serum bile acid levels, particularly when evaluating a fasting and post-prandial sample, can offer valuable diagnostic insights. Elevated fasting serum bile acids (FSBA) are indicative of hepatobiliary disease, with post-prandial samples providing additional diagnostic sensitivity, especially in cases of portosystemic shunts or portal vein hypoplasia. Ammonia levels also serve as an important diagnostic marker. High levels of ammonia indicate a dysfunction in the liver’s ability to detoxify by converting ammonia to urea, which is critical in diagnosing portosystemic shunting or significant hepatic dysfunction. PROTEIN C: A BIOMARKER OF HEPATIC FUNCTION Protein C, an anticoagulant protein synthesized in the liver, has emerged as a useful biomarker for hepatic function. Decreased activity of protein C has been found in dogs with congenital or acquired portosystemic shunts, offering a potential diagnostic tool to differentiate between portosystemic shunts and microvascular dysplasia (MVD). Low protein C levels, in conjunction with high bile acids, suggests the presence of portosystemic shunts and may guide further diagnostic imaging and treatment.
LIVER ENZYME TESTS: ALT, AST, AND THEIR IMPLICATIONS
One of the first indicators of hepatocellular injury are alterations in liver enzyme levels. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are two primary markers used to assess hepatocellular damage. ALT is more specific to hepatocytes and is a sensitive marker for liver injury, though it has a relatively long plasma half-life (~2.5 days) and may remain elevated for days to weeks following an acute insult. In contrast, AST is present in various tissues, particularly in mitochondria, and while it can indicate liver disease, it is less specific due to its presence in muscle tissue. AST also has a shorter half-life and normalizes more quickly (hours to days) compared to ALT. When ALT and AST levels rise, it is crucial to investigate the underlying cause, especially if the increase is greater than two times the normal reference range or persists for weeks to months. Elevated levels of these enzymes may indicate hepatocellular necrosis, viral hepatitis, or other liver pathologies.
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