Liver function tests measure various chemicals in the blood made by the liver. An abnormal result indicates a problem with the liver, and may help to identify the cause. Further tests may be needed to clarify the cause of the liver problem.
As the liver performs its various functions it makes a number of chemicals that pass into the bloodstream and bile. Various liver disorders alter the blood level of these chemicals. Some of these chemicals can be measured in a blood sample. Some tests that are commonly done on a blood sample are called 'LFTs' (liver function tests). These usually measure the following.
This is an enzyme that helps to process proteins. (An enzyme is a protein that helps to speed up chemical reactions. Various enzymes occur in the cells in the body.) Large amounts of ALT occur in liver cells. When the liver is injured or inflamed (as in hepatitis), the blood level of ALT usually rises.
This enzyme occurs mainly in liver cells next to bile ducts, and in bone. The blood level is raised in some types of liver and bone disease.
This is the main protein made by the liver, and it circulates in the bloodstream. The ability to make albumin (and other proteins) is affected in some types of liver disorder. A low level of blood albumin occurs in some liver disorders.
This measures albumin and all other proteins in blood.
This chemical gives bile its yellow/green colour. A high level of bilirubin in your blood will make you jaundiced ('yellow'). Bilirubin is made from haemoglobin. Haemoglobin is a chemical in red blood cells that is released when the red blood cells break down. Liver cells take in bilirubin and attach sugar molecules to it. This is then called 'conjugated' bilirubin which is passed into the bile ducts.
Other blood tests which may be done include:
The liver makes many of the proteins needed to make blood clot. In certain liver disorders the liver cannot make enough of these proteins and so blood does not clot so well. Therefore, blood clotting tests may be used as a marker of the severity of certain liver disorders.
This is another enzyme that occurs in liver cells. A high level of this enzyme is particularly associated with heavy alcohol drinking. (The liver breaks down and clears alcohol from the body and this enzyme is involved in the process.)
Blood tests may be done to detect:
Other types of scan, etc, may be needed to clarify the cause of a liver disorder, and/or to monitor its progress.
Alkaline phosphatase (ALP) – an enzyme related to the bile ducts; often increased when they are blocked
Aspartate aminotransferase (AST) – an enzyme found in the liver and a few other places, particularly the heart and other muscles in the body
Total bilirubin – measures all the yellow bilirubin pigment in the blood. Another test, direct bilirubin, measures a form made in the liver and is often requested with total bilirubin in infants with jaundice.
Albumin – measures the main protein made by the liver and tells how well the liver is making this protein
Total protein - measures albumin and all other proteins in blood, including antibodies made to help fight off infections
Other tests that can be used to assess liver function include gamma-glutamyl transferase (GGT), 5'- nucleotidase, and coagulation screen, together with bilirubin and urobilinogen in urine