Bile acids may be used diagnostically in dogs and cats in several ways.
In the body, bile acids function as an aid to intestinal absorption of fats in the diet, as well as fat-soluble vitamins. Bile is produced in the liver, passed through the liver in tiny channels called canaliculi, into the bile ducts, which pass through the pancreas, and into the small intestine.
From there, about 95% of the bile acids are reabsorbed into the bloodstream and returned to the liver for reuse. A molecule of bile acids may be used as many as 20 times before it is discarded in the stool.
Blood testing for bile acids is usually performed as “paired serum bile acids.” Under normal fasting conditions, serum bile acid levels are low, as little is required when no food is in the duodenum (the initial part of the small intestine). Therefore, from a fasted patient a blood sample is obtained and marked preprandial.” A “postprandial” sample is typically taken two hours after a meal of specific constitution.
For our laboratory, a preprandial sample should be less than 10 micromol\L to be considered normal. A postprandial level must not be higher than 20 micromol\L. Higher, and the implication is that the functionality of the liver is reduced, limiting its ability to reabsorb bile acids that have cycled down the intestinal tract and back to circulation.
Bile acids may also be measured in the urine. Normally, only small amounts of bile acids are eliminated in the urine, as 2-5% goes out in the stool. Therefore, if serum levels of bile acids increase above a threshold, one expects urine bile acids to increase as well. The test result is expressed as a ratio with urine creatinine (a waste product of the body) so as to eliminate variations in urine specific gravity (concentration). This test has rapidly risen in popularity as a single sample can be taken after a meal, and urine is easily obtained. Reliability of the test is high and has a strong correlation to the blood test described above.
Unlike AST, ALT, SAP and GGTP, the bile acids test is a true liver function test. Whereas the other tests tell us when the liver is damaged, bile acids studies tell us whether the liver is actually working, or not.
See you Monday, Dr. Randolph.