When it comes to examining a dog's liver, there are two ways of doing so—a bile acid test and a static enzyme level reading. The latter 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. As a healthy liver will "recycle" bile acids, a damaged liver will not. So a bile acid test examines if the liver has enough healthy cells to do its job if the liver has a good blood supply and if bile is moving through and out of the liver freely.
First, let's take a look at what bile acid actually is.
What are Bile Acids?
Bile is secreted by the liver and aids in digestion. When animals (as well as humans) 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 (portal) bloodstream and returned back to the liver. 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.
How is a Bile Acid Test Performed?
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. If the levels are high, there may be a problem with the liver or hepatic vasculature (the liver's blood flow).
What Do the Test Results Mean?
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. Bile acids are removed from liver (portal) blood by the liver cells (hepatocytes). If the liver cells are not functioning well, the bile acids remain in circulation and enter the body (systemic) blood supply where the elevated levels are measured by this test.
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.