Liver Function Tests

Liver enzyme tests, formerly called liver function tests (LFTs), are a group of blood tests that detect inflammation and damage to the liver. They can also check how well the liver is working. Liver enzyme testing includes ALT, AST, alkaline phosphatase; true liver function tests (LFTs) include PT, INR, albumin, and bilirubin.

Why Check Liver Function?

Most often, a liver test panel is ordered as a routine lab test. Many doctors check the liver in new patients or during annual physicals.

Your doctor may conduct liver enzyme and liver function tests 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)
• You drink alcohol excessively

Liver tests may be done together in a panel or tested separately.

Liver Enzyme Tests: ALT and AST

The liver filters and processes blood as it circulates through the body. It metabolizes 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 tests check the blood for two main liver enzymes:

• Aspartate aminotransferase (AST), formerly called SGOT. The AST enzyme is also found in muscles and many other tissues besides the liver.
• Alanine aminotransferase (ALT), formerly called SGPT. 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 Enzyme Tests: Alkaline Phosphatase, 5’ Nucleotidase, 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 gallbladder, under the liver. 

When bile flow is slow or blocked, blood levels of certain liver enzymes rise: 

• Alkaline phosphatase
• 5' nucleotidase
• Gamma-glutamyl transpeptidase (GGT) 

Liver tests may check for any or all of these enzymes in the blood. Alkaline phosphatase is by far the most commonly tested of the three. 

If alkaline phosphatase and/or 5' nucleotidase 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 gallbladder, or the tubes connecting them.