Liver function tests
Liver function tests (LFTs) are a group of blood tests that help to diagnose disorders of the liver. They can also check how well the liver is working. Liver function tests include alanine transaminase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), albumin, total protein and bilirubin.
Why check liver function?
LFTs may be performed as a routine blood test to help establish how healthy the liver is or to identify whether a liver disorder may be present and responsible for a person’s symptoms.
Your doctor may conduct LFTs if:
- You are taking a medication that can harm the liver.
- You have liver disease.
- You have symptoms of liver or bile system disease ( abdominal pain, nausea and vomiting, or yellow skin, for example).
- You drink alcohol excessively.
- You have been exposed to infections such as hepatitis B.
Liver function tests: ALT and AST
The liver filters and processes blood as it circulates through the body. It metabolises nutrients, detoxifies harmful substances, makes blood clotting proteins and performs many other vital functions. The cells in the liver contain proteins called enzymes that drive these chemical reactions.
When liver cells are damaged or destroyed, the enzymes in the cells leak out into the blood, where they can be measured by blood tests. Liver function tests check the blood for two main liver enzymes:
- Aspartate aminotransferase (AST). The AST enzyme is also found in muscles and many other tissues besides the liver.
- Alanine aminotransferase (ALT). ALT is almost exclusively found in the liver.
If ALT and AST are found together in elevated amounts in the blood, liver damage is most likely present.
Liver function tests: Alkaline phosphatase and GGT
Another of the liver's key functions is the production of bile, which helps digest fat. Bile flows through the liver in a system of small tubes (ducts), and is eventually stored in the gall bladder, under the liver.
When bile flow is slow or blocked, blood levels of certain liver enzymes may rise:
- Alkaline phosphatase
- Gamma-glutamyl transpeptidase (GGT)
Liver function tests may check for any or both of these enzymes in the blood.
If alkaline phosphatase and GGT are elevated, a problem with bile flow is most likely present. Bile flow problems can be due to a problem in the liver, the gall bladder or the tubes connecting them.