Liver function tests

Definition

Liver function tests generally refer to a group of blood tests that measure certain enzymes or proteins in your blood. Liver function tests are used to help detect, evaluate and monitor liver disease or damage.

The term "liver function tests" can be misleading, because not all of these tests actually measure liver function. Some tests, such as albumin and bilirubin, do. These liver function tests measure how well the liver is producing the protein albumin and clearing a blood waste product, bilirubin. Other tests commonly done, but not true "liver function" tests, measure enzymes that liver cells release in response to damage or disease.

Together, liver function tests help evaluate the overall health of your liver and can sometimes indicate other diseases, such as malnutrition or bone disease.

Why it's done

Liver function tests may be done for a variety of reasons, including:

• To screen for liver infections, such as hepatitis
• To monitor the progression of a disease like viral or alcoholic hepatitis and determine how well a treatment is working
• To measure the severity of a disease, particularly cirrhosis — an irreversible scarring of the liver
• To monitor possible side effects of certain medications

Liver function tests check the levels of certain enzymes and proteins in your blood. Higher or lower than normal levels can indicate liver problems. Some common liver function tests include:

• Alanine transaminase (ALT). An enzyme found mainly in liver cells, ALT helps your body metabolize protein. Normally, ALT levels in the blood are low. When the liver is damaged, ALT is released in the bloodstream and levels increase.
• Aspartate transaminase (AST). The enzyme AST plays a role in the metabolism of alanine, an amino acid. AST is found in high concentrations in liver cells. An increase in AST levels may indicate liver damage or disease.
• Alkaline phosphatase (ALP). ALP is an enzyme in the liver and bile ducts, as well as some other tissues. Higher than normal levels of ALP may indicate liver damage or disease.
• Albumin and total protein. Levels of albumin — a protein made by the liver — and total protein show how well your liver is making proteins that your body needs to fight infections and perform other functions. Lower than normal levels may indicate liver damage or disease.
• Bilirubin. Bilirubin results from the breakdown of red blood cells. Normally, bilirubin passes through the liver and is excreted in your stool. Elevated levels of bilirubin (jaundice) may indicate liver damage or disease.

Additional tests that may be used to evaluate liver function include:

• Gamma-glutamyltransferase (GGT). This test measures the amount of the enzyme GGT in the blood. Higher than normal levels may indicate liver or bile duct damage.
• L-lactate dehydrogenase (LDH). LDH is an enzyme found in many body tissues, including the liver. Elevated levels of LDH may indicate liver damage.
• Prothrombin time (PT). This test measures the clotting time of plasma. Increased PT may indicate liver damage.

Risks

Blood 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 from which your blood is drawn.

How you prepare

Certain foods and medications may affect the results of your liver function tests. Because of this, your doctor may tell you to fast for a period of time and avoid some medications before your blood is drawn. How much time you need to fast and what medications you avoid will depend on which liver function tests are being done. Your doctor will tell you what to avoid.

What you can expect

During the test
Liver function tests are done using a blood sample. Usually, the blood is drawn through a small needle inserted into a vein in the bend of your arm. You'll be asked to roll up your sleeve if you're wearing long sleeves. The person drawing the blood might tie a band around your upper arm and ask you to make a fist. This causes your vein to stand out more, making it easier to insert the needle into the vein. The needle is attached to a small tube, to collect your blood. All of this usually takes just a few minutes.

You may feel a quick pain as the needle is inserted into your arm and experience some short-term discomfort at the site after the needle is removed.

After the test
Once the needle is removed, a piece of gauze and a bandage or other similar material is used to cover the area where the needle was inserted. You'll be asked to gently apply pressure to the area for a minute or so, to help stop any bleeding.

Your blood will be sent to a laboratory for analysis. If the laboratory analysis is done on-site, you could have your test results within hours. If your doctor sends your blood to an off-site laboratory, you may receive the results within several days.

Results

Normal test results for typical liver function tests include:

• ALT. 7 to 55 units per liter (U/L)
• AST. 8 to 48 U/L
• ALP. 45 to 115 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 1.0 mg/dL
• GGT. 9 to 48 U/L
• LDH. 122 to 222 micromoles per liter (mcmol/L)
• PT. 8.3 to 10.8 seconds

These results are typical for adult men. Normal results may vary slightly from laboratory to laboratory and be slightly different for women and children. Results may also be affected by certain foods or medications. Be sure to mention any foods or medications you've eaten or taken so that your doctor can correctly interpret your results.

The farther from normal the test results are, the more likely you are to have significant liver disease. Your doctor will use these results to help determine if and what treatment is needed. If you already have liver disease, liver function tests can help determine how your disease is progressing and if you're responding to treatment.