Bile Acids

A marker of cholestasis used to assess the hepatobiliary system.

Bile acids are synthesized in the liver from cholesterol and enter the gallbladder as part of bile, where they accumulate and become concentrated. With food intake, they are released with bile into the intestine, where they emulsify fats, thereby facilitating digestion. In the intestine, 90% of delivered bile acids subsequently undergo reabsorption.

An increased fasting bile acid concentration is observed in disorders of bile outflow. After a meal, bile acid levels rise only slightly in healthy individuals but increase markedly in patients with various liver diseases, including cirrhosis, hepatitis, cholestasis, alcohol- and drug-induced liver injury, portal vein thrombosis, hepatic vein obstruction (Budd–Chiari syndrome), cholangitis, Wilson disease, and hemochromatosis. Metabolic disorders such as Gilbert syndrome, Dubin–Johnson syndrome, and Crigler–Najjar syndrome do not cause an increase in serum bile acid concentration. The test alone cannot differentiate among the causes of altered liver function and should always be used in conjunction with other methods of liver function assessment.

Measurement of bile acids is considered the most useful test for diagnosing intrahepatic cholestasis of pregnancy (manifested by pruritus, sometimes severe). This relatively rare, transient pathological condition is thought to be associated with increased steroid metabolism during pregnancy and a certain genetic predisposition. It affects approximately 1% of pregnant women and resolves immediately after delivery. In cholestasis of pregnancy, bile acid levels are often increased threefold or more (and may rise 10–100-fold above the usual reference range). A fasting bile acid concentration >40 μmol/L is associated with an increased risk of pregnancy complications. Aminotransferases may be within the normal range or moderately elevated. Alkaline phosphatase is almost always elevated (interpretation is complicated by a possible increase in the placental isoenzyme).

Bile acid testing may also be used to monitor liver status in patients with chronic hepatitis C, including as a marker of histological improvement in patients with chronic hepatitis C who achieve a sustained response to interferon therapy.