How to... interpret all those liver parameters!
The liver is a complex internal organ with a huge number of functions, including the metabolism of amino acids, lipids and carbohydrates, synthesis of cholesterol, clotting factors and plasma proteins, secretion of bilirubin, and bile, and detoxification/catabolism of some medications. Closely associated with the liver is the gallbladder; the main role of this organ is to store bile created by the liver.
There are a large number of different biochemistry tests we can use to evaluate the liver. The most common ones are:
ALT (alanine aminotransferase) and AST (aspartate transaminase) - these are enzymes which are released from damaged hepatocytes.
ALP (alkaline phosphatase) and GGT (gamma glutamyltransferase) are cholestatic enzymes. Elevations in these can be seen due to bile duct obstruction, metabolic defects in hepatocytes, and following administration of certain medications, or due to certain endocrinopathies. The classic example here is ALP and steroids - steroid administration can cause an elevation in ALP. Similarly, we can see an increased ALP in patients with Cushing's disease.
Total Bilirubin and Bile Acids are tests of liver function. These assess the liver's ability to carry out its normal function. For example, bile acids are stored in the gall bladder, and released into the intestine to aid in fat absorption. Following their use, they are transported back to the liver where they are reconjugated and excreted. An elevated bile acid level therefore indicates that the liver is unable to perform this function properly. Other tests of liver function include cholesterol, BUN and glucose, however these are not liver-specific, and elevations can be seen with other diseases and conditions.
References
Sirois, M. 2020. Laboratory Procedures for Veterinary Technicians, 7th edition. Elsevier.
