Liver function tests are blood tests used to help find the cause of your symptoms and monitor liver disease or damage. The tests measure the levels of certain enzymes and proteins in your blood.

Some of these tests measure how well the liver is performing its regular functions of producing protein and clearing bilirubin, a blood waste product. Other liver function tests measure enzymes that liver cells release in response to damage or disease.

Irregular liver function test results don't always mean liver disease. A member of your health care team will typically explain your results and what they mean.

Liver function tests can be used to:

• Screen for liver infections, such as hepatitis.
• Monitor a disease, such as viral or alcoholic hepatitis, and determine how well a treatment is working.
• Look for signs of serious disease, particularly scarring of the liver, called cirrhosis.
• Monitor possible side effects of medicines.

Liver function tests check the levels of certain enzymes and proteins in your blood. Levels that are higher or lower than usual can mean liver problems. The pattern and degree of elevation of these tests along with the overall clinical picture can provide hints to the underlying cause of these problems.

Some common liver function tests include:

• Alanine transaminase (ALT). ALT is an enzyme found in the liver that helps convert proteins into energy for the liver cells. When the liver is damaged, ALT is released into the bloodstream and levels increase. This test is sometimes referred to as SGPT.
• Aspartate transaminase (AST). AST is an enzyme that helps the body break down amino acids. Like ALT, AST is usually present in blood at low levels. An increase in AST levels may mean liver damage, liver disease or muscle damage. This test is sometimes referred to as SGOT.
• Alkaline phosphatase (ALP). ALP is an enzyme found in the liver and bone and is important for breaking down proteins. Higher-than-usual levels of ALP may mean liver damage or disease, such as a blocked bile duct, or certain bone diseases, as this enzyme is also present in bones.
• Albumin and total protein. Albumin is one of several proteins made in the liver. Your body needs these proteins to fight infections and to perform other functions. Lower-than-usual levels of albumin and total protein may mean liver damage or disease. These low levels also can be seen in other gastrointestinal and kidney-related conditions.
• Bilirubin. Bilirubin is a substance produced during the breakdown of red blood cells. Bilirubin passes through the liver and is excreted in stool. Higher levels of bilirubin might mean liver damage or disease. At times, conditions such as a blockage of the liver ducts or certain types of anemia also can lead to elevated bilirubin.
• Gamma-glutamyltransferase (GGT). GGT is an enzyme in the blood. Higher-than-usual levels may mean liver or bile duct damage. This test is nonspecific and may be elevated in conditions other than liver disease.
• L-lactate dehydrogenase (LD). LD is an enzyme found in the liver. Higher levels may mean liver damage. However, other conditions also may cause higher levels of LD.
• Prothrombin time (PT). PT is the time it takes your blood to clot. Increased PT may mean liver damage. However, it also can be higher if you're taking certain blood-thinning drugs, such as warfarin.

The blood sample for liver function tests is usually taken from a vein in your arm. The main risk associated with blood tests is soreness or bruising at the site of the blood draw. Most people don't have serious reactions to having blood drawn.

Certain foods and medicines can affect the results of your liver function tests. Your doctor will probably ask you to avoid eating food and taking some medicines before your blood is drawn.

During the test

The blood sample for liver function tests is usually drawn through a small needle inserted into a vein in the bend of your arm. The needle is attached to a small tube, to collect your blood. You may feel a quick pain as the needle is inserted into your arm and have some short-term soreness at the site after the needle is removed.

After the test

Your blood goes to a laboratory for analysis. If the lab analysis happens on-site, you could have your test results within hours. If your doctor sends your blood to an off-site laboratory, you may get the results within several days.

Standard range blood test results for typical liver function tests include:

• ALT. 7 to 55 units per liter (U/L).
• AST. 8 to 48 U/L.
• ALP. 40 to 129 U/L.
• Albumin. 3.5 to 5.0 grams per deciliter (g/dL).
• Total protein. 6.3 to 7.9 g/dL.
• Bilirubin. 0.1 to 1.2 milligrams per deciliter (mg/dL).
• GGT. 8 to 61 U/L.
• LD. 122 to 222 U/L.
• PT. 9.4 to 12.5 seconds.

These results are typical for adult men. Standard range results may be different from laboratory to laboratory. They also might be slightly different for women and children.

Your health care team uses the results to help diagnose your condition or decide on treatment you might need. Sometimes, additional blood tests and imaging may be used to help make a diagnosis. If you already have liver disease, liver function tests can help determine how your disease is progressing and if you're responding to treatment.