What are fasting serum bile acids (fS-Bile Acids)?
fS-Bile Acids (fasting serum bile acids), often referred to as TBA (Total Bile Acids), is a blood test that measures the concentration of bile acids in the bloodstream. Bile acids are produced in the liver from cholesterol and play a key role in fat metabolism, where they act as emulsifying agents and enable the breakdown and absorption of dietary fats and fat-soluble vitamins in the intestine.
After being synthesized in the liver, bile acids are stored in the gallbladder and released into the intestine in response to food intake. Most bile acids are then reabsorbed in the small intestine and transported back to the liver via the portal circulation in what is known as the enterohepatic circulation. Under normal conditions, this system is highly efficient, meaning that only small amounts of bile acids are present in the blood.
Elevated levels of bile acids in the blood occur when this circulation is disrupted, most commonly due to impaired liver function or obstruction of bile flow. Bile acids are therefore considered a sensitive marker of early liver dysfunction and cholestasis, sometimes rising before other liver function tests become abnormal.
Why are fS-Bile Acids analyzed?
Measurement of bile acids is primarily used to assess liver function and to identify conditions where bile flow is impaired, known as cholestasis. Because bile acid levels are influenced by food intake, the test is usually performed in a fasting state to ensure more reliable results.
- Evaluation of liver disease and early hepatic dysfunction
- Suspicion of cholestasis, meaning reduced or obstructed bile flow
- Diagnosis and monitoring of intrahepatic cholestasis of pregnancy (ICP)
- Complementary assessment alongside abnormal liver tests such as ALAT, ASAT and GT
- Investigation of symptoms such as pruritus (itching), particularly without rash, fatigue, or jaundice
Bile acids are particularly useful in suspected cholestatic conditions, as they often increase earlier than other liver biomarkers. This makes the test valuable for early detection.
How are fS-Bile Acids interpreted?
Low or normal levels of bile acids in the blood generally indicate normal hepatic uptake, synthesis, and excretion of bile acids. It also suggests that the enterohepatic circulation is functioning properly and that bile flow is not impaired.
Elevated levels indicate that the liver is not efficiently excreting bile acids into the bile, leading to accumulation in the bloodstream. This can occur in a range of conditions involving impaired bile flow or hepatocellular dysfunction.
Common causes of elevated bile acids include:
- Cholestatic liver diseases, where bile flow is impaired within or outside the liver
- Liver injury such as hepatitis, drug-induced liver injury, or non-alcoholic fatty liver disease (NAFLD)
- Intrahepatic cholestasis of pregnancy (ICP), where bile acids are often the most sensitive and early marker
During pregnancy, measurement of bile acids plays a particularly important role. Elevated levels are associated with an increased risk of fetal complications, which is why the test is commonly used in the evaluation of pruritus, especially affecting the palms and soles. Regular monitoring may be required in such cases.
It is important to interpret bile acid levels in conjunction with other laboratory findings and the patient’s clinical presentation. A single mild elevation does not necessarily indicate serious disease, whereas clearly elevated or rising levels over time may warrant further investigation.