The liver is the chemical centre of the human body. It helps regulate blood glucose, digest fats, and convert waste and toxins into harmless substances. When the liver is damaged, the enzymes and other chemicals contained within its cells are released into the blood stream. Levels of these substances that are out of the normal range may be indicative of liver damage or disease.
The liver helps in digestion, regulating blood sugar levels, and in cleansing the blood of harmful chemicals. It does this by releasing bile salts into the intestines, releasing stored glucose into the blood, and by using enzymes to transform toxic chemicals into their water-soluble, less toxic form, so they can then be excreted by the kidneys. When a problem with the liver is suspected, health care providers will order that levels of enzymes and other chemicals in the blood be tested.
Alanine aminotransferase (ALT), aspartate aminotransferase (AST), and alkaline phosphatase (ALP) are three enzymes that are present inside the cells of the liver. When the cells of the liver are damaged, the enzymes are measured in higher concentrations in the blood. The normal range for the three enzymes are: 5-43 International Units per litre (IU/L) for AST, 5-60 IU/L for ALT, and 30-115 IU/L for ALP.
Bilirubin is made by the liver as a byproduct of red blood cell decay. It is also an ingredient in bile salts released to the small intestine for the absorption of fat. If the liver is blocked or otherwise damaged, the levels of bilirubin in the blood increase, causing a yellow colour to the skin and eyes known as jaundice. The normal level of bilirubin in the blood is 0.2 milligrams per decilitre (mg/dl) to 1.5 mg/dL.
Albumin, globulins, and prothrombin are proteins created by the liver. Albumin helps in maintaining blood volume and blood pressure, while globulins and prothrombin help in the immune system and clotting, respectively. When the liver is damaged, it cannot create these proteins, so low levels are found in the blood. The normal level of albumin is 3.9 to 5 grams per decilitre (g/dl). Globulins are measured by the function of the immune system. Prothrombin is measured by the time it takes for a clot to form in the lab, which should be below 15 seconds.
Other tests performed during a liver function test may include gamma glutamyl transferase (GGT) and cholesterol. GGT is also found in the liver cells and is found in higher blood concentrations when the bile ducts are blocked. Cholesterol is made by the liver from dietary fat. High cholesterol is a sign that too much fat is being ingested. The normal level of GGT is 5 to 80 IU/L. Normal cholesterol should be below 200 mg/dL.