fS-Bile Acids (fasting serum bile acids), also referred to as TBA (Total Bile Acids), is a biomarker used to measure the concentration of bile acids in the blood. Bile acids are synthesized in the liver from cholesterol and play a central role in fat metabolism by facilitating the digestion and absorption of dietary fats and fat-soluble vitamins in the intestine.
Under normal physiological conditions, bile acids circulate between the liver and the intestine in an efficient system known as the enterohepatic circulation, where they are reused multiple times per day. Only small amounts enter the systemic circulation. When liver function is impaired or bile flow is obstructed, bile acids accumulate in the bloodstream, making this test a sensitive marker of disturbances in the liver and biliary system.
Elevated levels of bile acids are commonly seen in conditions where bile flow is impaired, known as cholestasis, but may also occur in other forms of liver dysfunction.
Mild elevations of bile acids are relatively common and do not necessarily indicate serious disease. Temporary increases may occur due to:
- Food intake: Bile acid levels naturally rise after meals, which is why the test is performed in a fasting state.
- Medications: Certain drugs may affect bile flow and liver function.
- Hormonal changes: For example, during pregnancy.
- Mild liver impairment: Such as in overweight individuals or early-stage fatty liver disease.
Persistently elevated or markedly increased bile acid levels may indicate:
- Cholestasis: Reduced or obstructed bile flow, either within the liver or in the biliary tract.
- Liver diseases: Such as hepatitis, non-alcoholic fatty liver disease (NAFLD/NASH), or cirrhosis.
- Drug-induced liver injury: Where bile excretion is impaired.
- Intrahepatic cholestasis of pregnancy (ICP): A condition in which bile acids are elevated and may cause symptoms such as pruritus.
What is a normal value for fS-Bile Acids?
The reference interval for fasting bile acids may vary slightly between laboratories, but a commonly used threshold is:
- Fasting: < 10 µmol/L
During pregnancy, specific thresholds are often applied. Levels above 10 µmol/L may raise suspicion of cholestasis, and higher levels typically require closer monitoring.
Elevated fS-Bile Acids
Elevated bile acid levels generally indicate that the liver is not efficiently excreting bile acids into the bile, resulting in their accumulation in the bloodstream. This is most commonly seen in cholestatic conditions but may also occur in hepatocellular injury.
During pregnancy, elevated bile acids are particularly important to identify, as they are associated with an increased risk of fetal complications. Symptoms such as pruritus, especially affecting the palms of the hands and soles of the feet, are a common indication for testing. A clear or progressive increase in bile acid levels over time should always prompt further investigation and complementary liver testing.
Low fS-Bile Acids
Low levels of bile acids in the blood generally have no clinical significance and are consistent with normal liver function. They indicate effective hepatic uptake and excretion of bile acids, as well as an intact enterohepatic circulation.
A low value is therefore considered a normal and desirable finding.
How are fS-Bile Acids interpreted in relation to other markers?
Bile acids are often analyzed alongside other liver biomarkers to provide a more comprehensive assessment of liver function:
- Bile acids and ALAT/ASAT: Concurrent elevations may indicate hepatocellular injury or inflammation.
- Bile acids and ALP: Elevated ALP together with bile acids is more suggestive of cholestasis or biliary tract involvement.
- Bile acids and GT: GT may support the suspicion of cholestasis or alcohol-related liver injury.
Measurement of bile acids is particularly relevant in the presence of symptoms such as:
- Pruritus without visible rash
- Jaundice (yellowing of the skin or eyes)
- Persistent fatigue
- Discomfort in the upper abdomen
Results should always be interpreted in the context of other laboratory findings, clinical symptoms, and medical history, as bile acids are a sensitive but not disease-specific marker of liver and biliary disorders.
