A biomarker is a measurable indicator of a biological condition. For the liver, these biomarkers are proteins or enzymes measured through blood tests, offering a window into the organ’s health and function. These tests are a primary method for investigating suspected liver disease, monitoring its activity, or as part of a routine check-up.
Common Markers of Liver Injury
When liver cells are damaged, substances they contain can spill into the bloodstream. Two of the most common enzymes measured to detect this are Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST). When liver cells are injured, their membranes can become “leaky,” allowing these enzymes to escape into the blood.
While both enzymes indicate liver cell damage, ALT is more specific to the liver. AST is also found in the heart, muscles, and kidneys, so an isolated elevation in AST is less conclusive. Because ALT is found in the highest concentrations within the liver, elevated levels strongly suggest the injury is liver-related.
Enzyme levels can increase within hours of liver damage and remain high for several days. Common causes of elevated ALT and AST include viral hepatitis, medication side effects, and non-alcoholic fatty liver disease. The degree of elevation does not always correlate with the extent of liver damage but is a sensitive signal that something is affecting the liver cells.
Biomarkers of Liver Function
Separate from markers of cellular injury, other biomarkers assess how well the liver is performing its jobs, like producing substances and clearing waste. This provides a different perspective, focusing on the functional performance of the organ rather than just the presence of damage.
Albumin is the most abundant protein in the blood, made exclusively by the liver to maintain fluid balance and transport substances. Since albumin has a half-life of about two to three weeks, a decrease in its levels can indicate a prolonged period of poor liver function, suggesting the liver’s synthetic capability is compromised.
Bilirubin is a yellow waste product from the breakdown of old red blood cells. A healthy liver processes bilirubin so it can be excreted in bile. If bilirubin levels rise, it may indicate the liver is struggling to process and clear it, which can be a sign of liver dysfunction or a bile duct blockage.
The Prothrombin Time (PT) test measures how long it takes for blood to clot. The liver produces most of the proteins needed for blood clotting, known as clotting factors. A prolonged PT can signify an impairment in the liver’s ability to synthesize these factors and can be affected within 24 hours of significant liver dysfunction.
Indicators of Bile Duct Obstruction
A different set of enzymes helps identify problems with the bile ducts, which carry bile from the liver to the small intestine. When these ducts are blocked or inflamed, it causes a condition known as cholestasis. The biomarkers for this issue are distinct from those indicating liver cell injury.
Alkaline Phosphatase (ALP) is an enzyme measured to detect bile duct problems. ALP is found in high concentrations in the cells lining the bile ducts. When a blockage occurs, pressure builds and causes the release of ALP into the bloodstream. These elevations are often more significant than the increases seen in ALT and AST during a cholestatic event.
ALP is not exclusive to the liver; it is also found in bone and the intestines. To confirm an elevated ALP is due to a liver or bile duct issue, Gamma-Glutamyl Transferase (GGT) is often measured alongside it. GGT is also found in the bile ducts but not in bone. If both ALP and GGT are high, it strongly suggests the problem originates in the hepatobiliary system.
Understanding Test Results
Abnormal liver biomarkers are not a diagnosis but a starting point for further evaluation. Because a single abnormal value may be temporary, healthcare providers look at the overall pattern of results and trends over time. They will interpret the findings in the context of your medical history, a physical exam, and other tests to determine the underlying cause.
The specific pattern of abnormalities provides clues. For instance, very high levels of ALT and AST with relatively normal ALP suggest direct injury to liver cells, a hepatocellular pattern. Conversely, significantly elevated ALP and GGT with milder elevations in ALT and AST point toward a cholestatic pattern. A mix of elevations can also occur, indicating a more complex injury pattern.
Common reasons for abnormal liver tests include non-alcoholic fatty liver disease (NAFLD), the most frequent cause in many developed countries. Other potential causes include:
- Alcohol consumption
- Viral infections like hepatitis B and C
- Side effects from medications
- Autoimmune conditions