Bile Acids

Indications for performing this test: Bile acids are useful for detection of congenital portosystemic shunts, identifying chronic liver damage prior to the onset of icterus, monitoring returning hepatic function after the onset of heptatic disease, as well as assessing acute hepatic injury. Other indications include: increased serum ALT and AST in a clinically ill patient, persistent increases in ALT in small animals and AST in the equine, juvenile animals with clinical signs of hepatic insufficiency, and in cats, significant and persistent elevations in AP that cannot be explained by non-hepatic disease.

CT scan through the liver performed with intravenous contrast to show multiple abnormal low attenuation areas suspicious of metastatic neoplasia. Abnormal bile acids may be detected.

Contraindications for performing this test: Bile acids will not provide any additional information if the animal is already showing signs of severe liver disease, such as icterus. Once the animal is showing signs of icterus, you should expect that bile acids to be elevated as well.

Pathophysiology: Bile acids produced by hepatocytes act as a biological detergent. In the duodenum, bile acids aid in the formation of micelles allowing for fats to be made soluble, digested, and absorbed. In a properly functioning liver, the bile acids are continually recycled via enterohepatic circulation.

Bile acids are synthesized from cholesterol, conjugated, and then excreted into the bile ducts. By the time the bile acids reach the ileum, they have been deconjugated by bacteria and are reabsorbed in portal circulation where they are efficiently removed for re-excretion. Only 5-10% of the bile acids are lost during a typical enterohepatic cycle. If any segment of this recycling loop is disrupted, there will be a change in the levels of circulating bile acids.

The Test: Assays for bile acids may be done either after fasting, 2 hours post-prandial, or both. The 2 hour post-prandial measurement following a period of fasting increases the sensitivity of the test, however abnormal fasting levels may often preclude the need for post-prandial evaluation. During periods of fasting, bile acids are extracted from the blood and are stored in the gall bladder, if the animal has one. During this time, we would expect serum levels to be low as long as hepatic uptake is functioning normally. After a meal, cholecystokinin release and contraction of the gall bladder results in increased amounts of bile in the small intestine. These high levels of acids then reach the portal vein and severely test the ability of the enterohepatic circulation system's ability to recycle them.

Special Considerations in the Horse: Normal serum bile acids in the horse is >2umol/L. However, icteric horses without liver disease may have serum bile acids up to 20umol/L. Elevation of liver bile acids in the horse in excess of 20umol/L indicates liver disease.