When it comes to examining a dog's liver, there are two main blood tests that can be used: a liver enzyme level and a bile acid test. The liver enzyme test can indicate the degree of inflammation or damage if any, while a bile acid test measures how the liver functions and if it is performing properly. A healthy liver will "recycle" bile acids, a damaged liver will not. A bile acid test examines if the liver is able to function to do it's job which in a way helps evaluate if enough healthy cells are present, if blood supply is adequate and if bile is appropriately moving through and out of the liver.
About Bile Acid
Bile is secreted by the liver and aids in digestion. When animals eat, they need the bile, along with other digestive elements secreted by the pancreas, to help break down foods, especially fats. The gallbladder, where the bile is stored, contracts to release bile into the small intestine as needed for digestion. From there, the bile acids do their work breaking down fats during the process of digestion.
The bile acids are then absorbed by the intestine into the liver, then into the bloodstream and returned back to the liver. While some stays in the intestine and is eventually lost in the stool. If the liver is functioning properly, the bile acids are removed from the bloodstream and returned to the gallbladder until they are needed again. This is called Enterohepatic Circulation and is the body's way of "recycling" the bile acids.
Performing the Test
In order to conduct a bile acid test, your dog will need to fast. Then blood is drawn and the dog is fed a fatty meal. Two hours later, the blood is drawn again. The blood tests measure pre- and post-meal levels of bile acids.
Interpreting the Results
Comparing the two blood levels, pre- and post-meal, allows the veterinarian to see how well the liver, bile ducts, and blood flow to the liver are functioning. If the liver cells are not functioning well, bile acids can remain in circulation and generating elevated levels.
If post-meal—or even in some cases, fasting—blood levels of bile acids are high, this means that the liver isn't doing its job of removing the bile acids from the blood as it should. The actual numbers that are considered "normal" vary with the laboratory used, so please discuss numerical lab findings with your vet.