What Is a Liver Function Test?
A liver function test is one of a group of tests that check levels of certain enzymes and other proteins in your blood.
Some of the tests look for enzymes that you’d find in your blood only when your liver is damaged or has a disease. Others check that the organ is working the way it should.
Your liver does all kinds of work that’s crucial for your health. It helps break down food, clean your blood, make proteins, and store energy. If something goes wrong with it, you might have a number of symptoms, from yellow skin to slurred speech. That’s when you might need a liver test.
Why Are Liver Function Tests Done?
You might have this test to check for a liver disease such as hepatitis (an illness in which your liver swells and can cause serious issues). General symptoms of liver problems include:
- Lack of appetite
- Nausea or vomiting
- Weakness or feeling very tired
- Yellowish eyes or skin (jaundice)
- Dark urine or light-colored poop (less common)
- Swelling in your belly (less common)
Often, however, you don’t notice any symptoms.
Your doctor may order a liver test if you’re more likely to have liver damage or disease, like if you:
- Drink heavily or have an alcohol use disorder
- Have a family member with liver disease
- Are very overweight, especially if you also have diabetes or high blood pressure
- Take medicine that can harm your liver
- Have gallbladder disease or anemia
The liver test can also help your doctor learn whether an illness is getting worse or how well treatment is working.
Types of Tests and What They Measure
There are a number of liver tests, with lots of complex names. Some common ones include:
- Alanine transaminase (ALT) test. ALT is an enzyme that helps break down proteins and is found mainly in your liver. High levels in your blood could mean you have liver damage.
- Alkaline phosphatase (ALP) test. ALP is an enzyme in your liver, bile ducts, and bone. You might have high levels if you have liver damage or disease, a blocked bile duct, or bone disease.
- Albumin and total protein test. Your liver makes two main proteins: albumin and globulin. Low levels might mean damage or disease. Keep in mind that your immune system also makes globulin.
- Aspartate transaminase (AST) test. AST is another enzyme in your liver. High blood levels could be a sign of damage or disease.
- Bilirubin test. Your body makes bilirubin when it breaks down red blood cells. Usually, your liver cleans bilirubin out of your body. If you have high levels in your blood, a problem called jaundice, you may have liver damage.
- Gamma-glutamyltransferase (GGT) test. High levels of the GGT enzyme could point to liver or bile duct damage.
- L-lactate dehydrogenase (LD) test. LD is another enzyme that’s high when you have liver damage, but other conditions can also raise its level.
- Prothrombin time (PT) test. This test measures how long it takes your blood to clot. If it takes a long time, that could be a sign of liver damage. Medications that thin your blood, such as warfarin (Coumadin), can also lead to a longer PT. You probably won’t have this test until you have had other liver tests.
How to Prepare for a Liver Function Test
Make sure to tell your doctor about any medicines you’re taking, including over-the-counter medications and herbal supplements. They may tell you to stop eating the night before the test.
What Happens During a Liver Function Test?
Your doctor uses a thin needle to take a small amount of blood, usually from your arm near your elbow. You’ll feel a pinch or stinging when the needle goes in.
You’ll probably have these tests more than once over a few days or weeks. Your doctor can look at how levels change to be sure of your condition.
If you get your test at a clinic that has a lab on site, you can get results in a few hours. If not, it might take a few days.
Liver Function Test Results
The results usually show normal values for the specific test, along with your numbers. Keep in mind that what’s normal can vary with different labs. It also depends on whether you’re male or female.
If your results aren’t normal, you’ll probably get more tests.
By itself, a liver test can’t tell for sure that you have a certain illness. Your doctor can help you understand what your results mean.
Liver Function Test Risks
Liver function tests are generally safe, with few risks. You may have some soreness or a bruise where the blood is taken. Call your doctor if it doesn’t get better after a couple of days.