October 2025 
In this VETgirl online veterinary continuing education blog, Dr. John Sessions, DACVIM of Nashville Veterinary Specialists reviews how to clinically interpret elevated liver enzymes. By examining ALT, AST, ALP, and GGT alongside functional tests and imaging, veterinarians can better identify underlying hepatocellular injury or cholestasis, improving diagnostic accuracy and guiding timely treatment decisions.

High ALT & ALP in Dogs: Ignore, Or Do All the Tests?

By John Sessions, DVM, DACVIM, Nashville Veterinary Specialists

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.

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.

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%.

Bone disease, especially in young, growing dogs or dogs with bone neoplasia, can also increase ALP levels. However, GGT remains unaffected by bone pathology, enhancing its diagnostic specificity. Similarly, anticonvulsant medications like phenobarbital may increase ALP but do not affect GGT activity.

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.

Imaging and Biopsy: Tools for Liver Diagnosis

Imaging techniques, including ultrasonography and radiographs, are essential for evaluating liver size, shape, and the presence of masses or biliary system abnormalities. However, these methods are not sufficient to differentiate specific types of parenchymal liver changes. Fine needle aspiration (FNA) is useful for identifying vacuolar hepatopathies or liver neoplasia but is less effective in detecting inflammatory changes.

Liver biopsy remains the gold standard for diagnosing the type and extent of hepatic damage. Surgical and ultrasound-guided needle biopsies provide definitive histological information, allowing for targeted treatment. A study by Dr. David Twedt evaluating 150 liver biopsies in dogs revealed chronic hepatitis, neoplasia, and vascular anomalies as common causes of liver disease. However, the study also indicated that a needle biopsy should be interpreted with caution due to variable correlations with wedge biopsy results.

Canine Chronic Hepatitis

Chronic hepatitis in dogs is a condition characterized by persistent inflammation of the hepatic parenchyma, which, if left untreated, can progress to cirrhosis and portal hypertension. Laboratory findings typically include elevated ALT and AST levels, with some dogs showing increased bilirubin and decreased albumin levels as the disease progresses. Serum bile acids are often elevated in chronic hepatitis, with post-prandial measurements offering additional diagnostic utility.

The etiology of chronic hepatitis remains unclear in most cases, although copper-associated hepatopathy, particularly in Bedlington terriers, is well-documented. Copper accumulation in the liver is a significant concern, and diagnosis is made through liver biopsy and histochemical staining techniques. Other potential causes include drug-induced hepatitis, immune-mediated conditions, and infections like leptospirosis or Bartonella.

Management of Chronic Hepatitis

Treatment of chronic hepatitis aims to address the underlying cause (if identified) and to manage the inflammatory response to prevent further hepatic damage. Specific therapeutic interventions may include anti-inflammatory drugs like prednisone, azathioprine, or cyclosporine. In cases of copper-associated hepatitis, copper chelation with agents such as penicillamine or trientine is critical, followed by lifelong treatment with zinc to prevent copper re-accumulation.

Supportive care, including the use of antioxidants such as Vitamin E, SAMe, and milk thistle, is commonly employed to protect hepatocytes from oxidative damage. Managing secondary complications like ascites, hepatic encephalopathy, and GI ulceration is also essential in cases of advanced liver disease.

Prognosis and Clinical Outcome

The prognosis for dogs with chronic hepatitis depends largely on the stage of the disease and the presence of secondary complications like cirrhosis or portal hypertension. Studies show that dogs with early-stage chronic hepatitis can live for several years with appropriate treatment, while those with advanced cirrhosis often have a much shorter survival time. Prognosis is particularly poor for dogs with significant hypoalbuminemia, hypoglycemia, and coagulopathies.

Conclusion

Liver diseases in veterinary medicine encompass a range of pathologies, each with distinct diagnostic and management strategies. Early diagnosis, aided by a combination of enzyme tests, bile acids, imaging, and biopsy, is critical for successful treatment and improved prognosis. With advances in diagnostic tools, such as protein C activity and imaging techniques, veterinarians can now more accurately assess liver function and tailor treatment to the specific needs of their patients.

The key to managing liver disease lies in a comprehensive approach that incorporates clinical evaluation, laboratory testing, and appropriate imaging techniques, along with a focus on disease-specific therapies. With continued research and clinical experience, veterinarians can offer improved care and outcomes for dogs with hepatic diseases.

 

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