Liver function tests help determine the health of your liver by measuring the levels of proteins, liver enzymes, or bilirubin in your blood.
A liver function test is often given in the following situations:
- to screen for liver infections, such as hepatitis C
- to monitor the side effects of certain medications known to affect the liver
- if you already have a liver disease, to monitor the disease and how well a particular treatment is working
- to measure the degree of scarring (cirrhosis) on the liver
- if you’re experiencing the symptoms of a liver disorder
- if you’re planning on becoming pregnant
Many tests can be performed on the liver, but most of them don’t measure the overall function of the liver. Commonly used tests to check liver function are the alanine transaminase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), albumin, and bilirubin tests. The ALT and AST tests measure enzymes that your liver releases in response to damage or disease. The albumin and bilirubin tests measure how well the liver creates albumin, a protein, and how well it disposes of bilirubin, a waste product of the blood.
Having abnormal results on any of the liver function tests doesn’t necessarily mean you have liver disease or damage. Talk to your doctor about the results of your liver function test.
Liver function tests are used to measure specific enzymes and proteins in your blood. Depending on the test, either higher- or lower-than-normal levels of these enzymes or proteins can indicate a problem with your liver.
Some common liver function tests include:
Alanine transaminase (ALT) test
Alanine transaminase (ALT) is used by your body to metabolize protein. If the liver is damaged or not functioning properly, ALT is released into the blood. This causes ALT levels to increase. A high result on this test can be a sign of liver damage. The normal range for ALT is 7–55 units per liter (U/L), according to the Mayo Clinic. Low ALT is not indicative of any health issues.
Aspartate aminotransferase (AST) test
Aspartate aminotransferase (AST) is an enzyme found in several parts of your body, including the heart, liver, and muscles. Since AST levels aren’t specific for liver damage, it’s usually measured together with ALT to check for liver problems. Your doctor may use an ALT-to-AST ratio to help with their diagnosis. When the liver is damaged, AST is released into the bloodstream. A high result on an AST test might indicate a problem with the liver or muscles. The normal range for AST is 8–48 U/L. Low AST is not indicative of any health issues.
Alkaline phosphatase (ALP) test
Alkaline phosphatase (ALP) is an enzyme found in your bones, bile ducts, and liver. An ALP test is typically ordered in combination with several other tests. High levels of ALP may indicate liver damage, blockage of the bile ducts, or a bone disease.
Children and adolescents may have elevated levels of ALP because their bones are growing. Pregnancy can also raise ALP levels. The normal range for ALP is 45–115 U/L.
Low levels of ALP can occur following a blood transfusion or heart bypass surgery. Low ALPcan also result from a variety of conditions, including zinc deficiency, malnutrition, and Wilson disease.
Albumin is the main protein made by your liver. It performs many important bodily functions. For example, albumin:
- stops fluid from leaking out of your blood vessels
- nourishes your tissues
- transports hormones, vitamins, and other substances throughout your body
An albumin test measures how well your liver is making this particular protein. A low result on this test indicates that your liver isn’t functioning properly. The normal range for albumin is 3.5–5.0 grams per deciliter (g/dL).
A damaged liver can’t properly process bilirubin. This leads to an abnormally high level of bilirubin in the blood. A high result on the bilirubin test indicates that the liver isn’t functioning properly. The normal range for bilirubin is 0.1–1.2 milligrams per deciliter (mg/dL).
Liver tests can help determine if your liver is working correctly. The liver performs a number of vital bodily functions, such as:
- removing contaminants from your blood
- converting nutrients from the foods you eat
- storing minerals and vitamins
- regulating blood clotting
- producing proteins, enzymes, and bile
- making factors that fight infection
- removing bacteria from your blood
- processing substances that could harm your body
- maintaining hormone balances
Problems with the liver can make a person very sick and can even be life-threatening.
Symptoms of a liver disorder include:
- fatigue or loss of energy
- weight loss
- jaundice (yellow skin and eyes)
- symptoms of nephritic syndrome (swelling around the eyes, belly, and legs)
- discolored bodily discharge (dark urine or light stools)
- abdominal pain
Your doctor may order a liver function test if you’re experiencing symptoms of a liver disorder or even if you’re planning to become pregnant. The different liver function tests can also check for infection, monitor the progression or treatment of a disease, and test for the side effects of certain medications.
Your doctor will give you complete instructions on how to prepare for the blood sample portion of the test. Certain medications and foods may affect levels of these enzymes and proteins in your blood. Your doctor may ask you to avoid some types of medications, or they may ask you to avoid eating anything for a period of time before the test.
You may want to wear a shirt with sleeves that can easily be rolled up to make it easier to collect the blood sample.
You may have your blood drawn in a hospital or at a specialized testing facility. To administer the test:
- The healthcare provider will clean your skin before the test to prevent any microorganisms on your skin from contaminating the test.
- They’ll likely wrap a cuff or some sort of pressure device on your arm. This will help your veins become more visible. They will use a needle to draw several samples of blood from your arm.
- After the draw, the healthcare provider will place some gauze and a bandage over the puncture site. Then they will send the blood sample to a laboratory for testing.
Blood draws are routine procedures and rarely cause any serious side effects. However, the risks of giving a blood sample include:
After the test, you can usually leave and go about your life as usual. However, if you feel faint or lightheaded during the blood draw, you should rest before you leave the testing facility.
The results of these tests may not tell your doctor exactly which condition you have or the degree of any liver damage, but they might help your doctor determine the next steps. Your doctor will call you with the results or discuss them with you at a follow-up appointment.
In general, if your results indicate a problem with your liver function, your physician will review your medications and your past medical history to help determine the cause. If you’re a heavy drinker of alcohol, then you’ll need to stop drinking. If your doctor identifies that a medication is causing the elevated liver enzymes, then they will advise you to stop the medication.
Your doctor may decide to test you for hepatitis. They may also choose to do imaging, like an ultrasound or CT scan, to evaluate the liver for fibrosis or fatty liver disease.