The liver, located in the right upper portion of the abdominal cavity just beneath the right side of the rib cage, has many vital functions. Briefly, some of these functions are:
- Detoxification of blood
- Production of important clotting factor and other important proteins
- Metabolizing (processing) medications and nutrients
- Processing of waste products of hemoglobin
- Storing of vitamins, fat, cholesterol, and bile
- Production of glucose
In general, liver blood tests are used to detect an injury or an inflammation to the liver. These tests are commonly ordered and performed in many situations, such as in routine health screening, evaluation of abdominal pain, or suspected liver disease. The liver blood tests are typically done as a part of the comprehensive metabolic panel which also includes electrolyte levels and kidney function.
Liver blood tests are some of the most commonly performed blood tests. These tests can assess liver functions or liver injury. An initial step in detecting liver damage is a simple blood test to determine the presence of certain liver enzymes (proteins) in the blood.
Under normal circumstances, these enzymes reside within the cells of the liver. But when the liver is injured for any reason, these enzymes are spilled into the blood stream. Enzymes are proteins that are present throughout the body, each with a unique function. Enzymes help to speed up (catalyze) routine and necessary chemical reactions in the body.
Among the most sensitive and widely used liver enzymes are the aminotransferases. They include aspartate aminotransferase (AST or SGOT) and alanine aminotransferase (ALT or SGPT). These enzymes are normally contained within liver cells. If the liver is injured or damaged, the liver cells spill these enzymes into the blood, raising the enzyme levels in the blood and signaling liver disease.
The main values measured in liver blood tests are the aminotransferases (alanine aminotransferase or ALT and aspirate aminotransferase or AST). The other measurements include alkaline phosphate, albumin, and bilirubin. It is important to note that these tests are commonly referred to as “liver function tests”, but this term is misleading as the aminotransferases and alkaline phosphatase do not reflect the function of the liver. Strictly speaking, the true liver function tests (LFT’s) include albumin, bilirubin, blood coagulation panel, and glucose.
More specifically, AST, ALT, and alkaline phosphatase are called the liver enzymes and they typically are used to detect damage or injury to the liver (not its function)
AST (SGOT) is normally found in a variety of tissues including liver, heart, muscle, kidney, and brain. It is released into the serum when any one of these tissues is damaged. For example, its level in serum rises in heart attacks or with muscle disorders. It is therefore, not a highly specific indicator of liver injury as it can occur from other injured tissues.
ALT (SGPT) is, by contrast, normally found largely in the liver. This is not to say that it is exclusively located in liver, but that is where it is most concentrated. It is released into the bloodstream as the result of liver injury. Thus, it serves as a fairly specific indicator of liver status.
Abnormal liver tests may be detected in the blood in a variety of liver conditions:
- Mild to moderate elevations of the liver enzymes are common.
- Chronic hepatitis B and hepatitis C is a cause of chronic mild to moderate liver enzyme elevations.
- Chronic and acute alcohol use is also a common cause of abnormal liver tests.
- Some medications can cause mild to moderate increase in the liver enzymes.