Liver function blood tests check for liver disease or damage such as cirrhosis, chronic liver failure or hepatitis. A doctor may also use the tests to monitor how the liver responds to treatment. Some liver function blood tests help diagnose other diseases such as malnutrition or bone disorders.
Significance
The liver helps the body create proteins, blood clotting factors, enzymes and aids in keeping hormones balanced. It also processes and moves substances through the body that could be harmful. Liver function blood tests help pinpoint a diagnosis or help monitor treatment for conditions when one or more of these processes are disrupted. People at risk for liver conditions include heavy drinkers, those who may have been exposed to hepatitis, those with a family history of liver disease and those who may abuse drugs.
Blood Sample
To conduct the tests, a doctor orders a blood sample, usually taken from a vein in the arm or wrist by way of a needle. This can produce bruising or soreness at the site. Blood may also be taken from the heel using a heelstick. This method, usually done on babies, uses a small blade to cut the skin to draw blood from the heel. Some tests may also use a urine sample instead of blood.
Test Measurements
Depending on the type of test, a doctor measures different kinds of enzymes or proteins in the blood to assess how well the liver is working. Measurements below or above a certain range may indicate a liver problem.
Types
There are several common types of liver function blood tests. A doctor may order all of these tests together using one blood sample, called a liver panel:
Alanine transaminase, or ALT, is an enzyme in the liver that helps the body process proteins. When the liver is damaged--by diseases such as hepatitis, for example--it releases a large amount of ALT into the bloodstream. An ALT test measures the amount of this enzyme in the bloodstream. A normal ALT measurement is 7 to 55 units per liter (U/L). A test result above this range may be a sign of a liver condition.
Aspartate transaminase, or AST, is another liver enzyme that helps break down amino acids. Healthy people have low AST levels, with normal levels ranging from 8 to 48 U/L. Higher levels in the blood may be a sign of liver damage or disease.
An enzyme called alkaline phosphatase, or ALP, lives in the bile ducts, which are tubes where bile travels from the liver to the bowel. Bile helps break down fat. A normal ALP measurement is 45 to 115 U/L. Higher levels may indicate blocked bile ducts. The test can also help in diagnosing cirrhosis, liver cancer, hepatitis and liver poisoning due to drugs. ALP is also found in the bones, and an ALP test may help diagnose bone disorders as well.
The liver makes a protein called albumin, which performs important functions such as transporting hormones, fatty acids and other substances to various parts of the body. A total protein test measure albumin as well as other kinds of proteins. Normal albumin levels range from 3.5 to 5.0 grams per deciliter (g/dL), while normal total protein levels range from 6.3 to 7.9 g/dL. Low albumin or other protein levels may indicate a liver condition.
The body constantly makes and breaks down red blood cells. When the body breaks down these cells, it leaves behind a substance called bilirubin. Bilirubin is considered a waste, so the body normally gets rid of it by passing it through the liver and out through the stool. If the liver is damaged, bilirubin levels may build up. Healthy bilirubin levels range from 0.1 to 1.0 milligrams per deciliter (mg/dL). Blood tests that show higher levels may indicate liver damage. A doctor orders this test to monitor symptoms of liver damage, such as jaundice. Jaundice results when bilirubin builds up in the blood, causing yellowing of the skin and eyes.
Considerations
Normal test result ranges listed above are typical for adult men. Normal ranges for women and children may be different. A doctor may have a person fast before a test, as some foods could affect results. Certain medicines might also influence a test's outcome, so following a doctor's instructions before testing is key.
