What Are Liver Function Tests and Why Do You Need Them?
Doctors may order liver function tests as a way to check a person's liver for injury, infection, or disease. Liver function tests are a type of blood test, and they're usually (but not always) done as a group. You may hear your doctor refer to these tests by their medical name, hepatic function panel or liver profile.
If your liver isn't working properly, it can affect your overall health. That's because the liver plays lots of different roles the body, such as storing fuel from food, making proteins the body needs, and helping to remove toxins.
Your doctor may send you for liver function tests if you have signs of liver disease — including jaundice (yellowish skin or eyes), dark urine, nausea, vomiting, or abdominal (belly) pain or swelling.
Your doctor may order liver function tests to help diagnose a viral infection involving the liver (like mono or hepatitis, for example) or to look for possible effects of cancer or other diseases on the liver. Doctors also use liver function tests to monitor people who are taking medicines that might cause liver-related side effects.
Several different tests are in the liver function panel. Your doctor may order one, a couple, or all of these tests. The good news is you should only need to get blood taken once. The lab uses the same blood sample to run all the tests.
The different tests that make up a liver function test are:
- Alanine aminotransferase (ALT, also known as SGPT). This enzyme plays a role in processing proteins. When the liver is injured or inflamed, levels of ALT in the blood usually rise.
- Aspartate aminotransferase (AST, also known as SGOT). This enzyme is found in several body tissues, including the liver. Like ALT, AST also plays a role in processing proteins. If the liver is injured, the body releases AST into the bloodstream.
- Alkaline phosphatase (ALP). This enzyme is found in several body tissues, including the liver. Kids and teens normally have higher levels of ALP than adults because of bone growth. But ALP levels that are higher than normal can be a sign of liver diseases or blocked bile ducts.
- Total bilirubin and direct bilirubin. Bilirubin is a byproduct of the normal breakdown of red blood cells. It usually passes through the liver and is flushed from the body. But if that doesn't happen due to a liver disease, bilirubin levels in the blood can rise and the skin can take on the yellow discoloration known as jaundice. Tests for bilirubin may be total (measuring the level of all of the bilirubin in the blood) or direct (measuring only bilirubin that has been processed by the liver and attached to other chemicals).
- Albumin and total protein. Liver function tests include measuring albumin (the major blood protein produced by the liver), as well as the total amount of all proteins in the blood. When there's a problem with the liver, there can be changes in the amounts of albumin and other proteins it produces.
Although liver function tests can be done without any special preparation, the results are more accurate when a person has been fasting. So your doctor may ask you not to eat or drink for 8 to 12 hours before having your blood taken.
Tell your doctor about any medications (including over-the-counter medicines or herbal supplements) you're taking because certain drugs might alter the test results.
It can help to wear a T shirt or other short-sleeve top on the day of the test to make things faster and easier for the technician who will be drawing the blood.
A health professional will usually draw the blood from a vein in your arm — most often on the inside of the elbow, but sometimes on the back of the hand. The technician cleans the skin surface with antiseptic and ties an elastic band (tourniquet) around the upper arm so the veins swell with blood and are easy to see.
Next, it's time for the needle. It should feel like a quick pinprick. Occasionally, it can be hard to find a vein so a nurse, doctor, or technician may need to try more than once. That's not the norm, though — most people's veins are easy to find.
It's best to try to relax and stay still during the procedure since tensing muscles can make it harder and more painful to draw blood. And if you don't want to watch the needle being inserted or see the blood collecting, you don't have to. Look the other way and maybe relax by focusing on saying the alphabet backwards, doing some breathing exercises, thinking of a place that makes you happy, or listening to your favorite music.
The technician will draw the blood so it collects in a vial or syringe. Collecting blood will only take a few minutes. Once the technician has enough blood, he or she removes the needle and covers the area with cotton or a bandage to stop the bleeding. After the test, you may notice some bruising — that's normal and it should go away in a few days.
Don't be afraid to ask the technician if you have any questions about the blood draw.
A blood test is a safe procedure and there are no real risks. Some people may feel faint or lightheaded during a blood test. And, while nobody really loves needles, a few teens have a strong fear of them. If that's you, talk to your doctor since there are ways to make the procedure easier for you.
The blood sample will be processed by a machine. It usually only takes a few hours or a day or so for your doctor to get the results. If the results seem to point to a possible liver problem, your doctor may want to do other tests to find out what the cause is and how to treat it.
Reviewed by: Steven Dowshen, MD
Date reviewed: March 2011
Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor.
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