An enzyme test is a blood test that measures certain enzyme levels to assess how well the body’s systems are functioning and whether any tissue damage has occurred. Liver enzyme tests are used to diagnose and monitor the extent of liver diseases or disorders. Cardiac enzyme tests are used to diagnose a heart attack and determine the extent of damage to the heart muscle.

An enzyme test will likely be performed in a physician’s office. During the test, blood is drawn from a vein or capillary in the heel, finger, toe, earlobe, back of the hand or inside the elbow. Enzyme tests typically do not cause much pain for the patient, although a stinging sensation may be felt when the needle is inserted. After the blood is collected, the needle is withdrawn and a small cotton ball or pad is applied with light pressure over the puncture site.

The sample obtained during the test is sent to a laboratory for analysis. Once the results come in, the physician will discuss them with the patient. Appropriate treatment will then follow if necessary.

There are three factors that may interfere with the enzyme test results: vigorous exercise, certain medications and (in some cases) eating before the test. Patients are strongly encouraged to follow their physician’s orders in order to maximize the accuracy of their test results.

Enzyme tests use a blood sample to measure certain enzyme levels and assess how well the body’s systems are functioning and whether there has been any tissue damage. Enzymes are found in many body tissues, including the liver, heart, muscles, lungs and pancreas.

Liver enzymes are enzymes that help perform chemical reactions in the body necessary to normal liver functions. Healthy individuals have low levels of these enzymes in their blood. However, higher or lower levels of these enzymes may indicate the presence of disease, including digestive system disorders.

Liver enzyme tests may be recommended for patients who are at high risk of liver disease or who are experiencing symptoms of liver disease, such as fatigue, jaundice, abdominal pain, fluid retention, confusion and gastrointestinal bleeding.

Conditions that may cause elevated levels of liver enzymes include:

• Use of certain medications, including nonsteroidal anti-inflammatory drugs (NSAIDs), cholesterol-lowering medications, antibiotics and anti-seizure medications.
  
  
• Autoimmune disorders of the liver and bile ducts, including autoimmune hepatitis, primary sclerosing cholangitis and primary biliary cirrhosis.
  
  
• Metabolic liver diseases such as hemochromatosis and Wilson's disease.
  
  
• Tumors of the liver and bile ducts.
  
  
• Infections such as viral hepatitis and mononucleosis.
  
  
• Heavy alcohol consumption.
  
  
• Obesity.
  
  
• Diabetes.
  
  
• Excessive use of vitamins and herbal supplements.

Other specific enzymes may be used to determine the severity of damage to an organ. For instance, amylase, a pancreatic enzyme, is measured in the blood to diagnose pancreatitis (inflammation of the pancreas), which may be the cause of abdominal pain.

There are several different types of enzyme tests that may be administered to help diagnose disease. The most commonly performed liver enzyme tests include:

• Aspartate aminotransferase (AST). AST is found in high concentrations in liver cells, heart muscle and skeletal muscle cells. Although AST is not specific to liver disease, levels are a vital component of liver enzyme testing because it is the best enzyme to use in tracking both the tissue damage and the healing process. Diseases that impact liver cells may cause the release of AST.
  
  A maximum increase of 20 times greater-than-normal AST levels usually indicates severe viral hepatitis, severe trauma or surgery, whereas AST levels that are 10 to 20 times greater than normal may indicate a heart attack or alcoholic hepatitis. Moderate to high levels of five to 10 times greater than normal could be caused by muscular dystrophy or chronic hepatitis. Other disorders that may be indicated by high AST levels include acute hemolytic anemia, acute pancreatitis, acute kidney failure, hepatic necrosis (liver tissue death), liver cancer, recent convulsion, deep burn or skeletal muscle trauma. Acute liver injury secondary to alcohol use (alcoholic hepatitis) usually causes AST elevation.
  
  AST levels that are two to five times lower than normal may indicate a number of conditions, including pulmonary embolism and disorders of the liver or pancreas.
  
  
• Alanine aminotransferase (ALT), formerly known as serum glutamate pyruvate transaminase (SGPT). ALT is one of two enzymes that convert amino acids to amino acid residues, which are vital to energy production. It is found in a number of tissues, but its highest levels of concentration are found in the liver. When the liver is injured, these enzymes are released into the blood.
  
  Higher-than-normal levels of ALT may indicate the presence of hepatic ischemia (deficiency of blood or oxygen supply to the liver) or liver tumor. Moderate levels indicate hepatitis. Moderate to high levels indicate chronic hepatitis or severe liver congestion due to heart failure. Very high ALT levels are an indication of severe hepatitis (inflammation of the liver) caused by infection or medications. Low levels occur during a heart attack.
  
  
• Alkaline phosphatase (ALP). An enzyme found primarily in the liver, bones, kidneys and intestinal lining that influences bone growth and the transport of fats (lipids) through the bloodstream. ALP increases may be caused by blood disorders or liver disease and, in combination with other enzyme tests, ALP levels can be used to differentiate between the two. Children have higher levels than adults because their bones are still growing. Pregnant women also have greater-than-normal levels of ALP.
  
  Fat in the liver secondary to obesity, diabetes, hyperlipidemia (elevated fat levels in the blood) or other reasons can cause higher-than-normal ALP levels. Higher-than-normal ALP levels may indicate diseases such as anemia, primary biliary cirrhosis, hepatitis and hyperparathyroidism (excessive production of parathyroid hormone). Such levels also may indicate bone disorders such as leukemia (bone marrow disease), osteomalacia (softening of the bones), Paget’s disease (a metabolic bone disease) and rickets (childhood softening of the bones). Lower-than-normal levels may indicate malnutrition or protein deficiency.
  
  
• Gamma transferase glutamyl, also called gamma glutamyl transpeptidase (GGT). An enzyme found primarily in the kidneys that is involved in the transfer of amino acids across cell membranes. The test can be used to reveal disorders of the liver, bile ducts and kidneys. GGT levels are measured in combination with other tests.
  
  Combined with an elevated ALP, a normal GGT level indicates that the ALP elevation is being caused by a bone disorder rather than liver dysfunction. An elevated GGT level five to 10 days following a heart attack is either a sign of the healing process or that the weakened heart is affecting liver functions. Other conditions that may be indicated by greater-than-normal levels of GGT include congestive heart failure, cholestasis, cirrhosis, liver tumor and hepatitis.

A variety of factors can influence liver enzyme levels including drugs, tumors, infections, toxins or trauma. When liver damage occurs, the enzymes are released in stages. When all of these enzyme levels begin to decline, it is a sign that the damaged liver is beginning to heal. Measuring a single enzyme level will not diagnose a condition, but a combination can provide valuable information to help in the diagnosis and management of various conditions.

Enzyme testing may be conducted either as part of making an initial diagnosis or to monitor the progress of treatment for an existing disorder. Some conditions will require a series of regular enzyme tests over time.

A muscle enzyme test measures the blood levels of specific muscle enzymes. Muscle enzymes are vital in the chemical reactions that allow muscles to function normally. The two most commonly performed muscle enzyme tests include:

• Aldolase. An enzyme that is involved in the breakdown of glucose, fructose and galactose – a process used by cells to generate energy.
  
  
• Lactate dehydrogenase (LDH). An enzyme that helps convert lactic acid to pyruvic acid – an essential step in the metabolic processes, which ultimately produces cellular energy.

A cardiac enzyme test measures the blood levels of specific cardiac enzymes. Cardiac enzymes help the natural chemical reactions that allow the heart to function normally. The two most common cardiac enzyme tests performed are:

• Creatine kinase (CK). CK adds a phosphate group to creatine, turning it into the high-energy molecule phosphocreatine. The body’s cells use phosphocreatine as a quick source of energy.
  
  
• Cardiac troponin. Troponin is a calcium-regulated protein found in muscle tissue that controls the interactions of two other substances (actin and myosin) that cause the heart muscle to contract or squeeze.

Other enzymes commonly tested in the blood include:

• Amylase (AML). Also known as alpha–amylase, AML is an enzyme that helps digest glycogen and carbohydrates. AML is produced mainly in the salivary glands and pancreas.
  
  
• Angiotensin-converting enzyme (ACE). This enzyme is produced by endothelial cells (cells lining the interior surface of blood vessels) to help regulate blood pressure. ACE is produced throughout the body, but is especially concentrated in the lungs.
  
  
• Lipase. This enzyme is produced in the pancreas and is released into the digestive tract to help digest fatty foods. Serum levels of lipase (like amylase) may be elevated with pancreatitis

Enzyme tests are likely to be performed in a physician’s office. Patients may be asked to refrain from eating for six hours prior to the test. In addition, they may be instructed to stop using alcohol and certain medications, which may include antibiotics, narcotics, tricyclic antidepressants, oral contraceptives (birth control pills), arthritis medications, anti-diabetic drugs, anti-inflammatories, anticonvulsants, antihypertensives, antiemetics and androgens (sex hormones). Other drugs that may be restricted include:

• Allopurinol (gout medication)
• Clofibrate (cholesterol drug)

During an enzyme test, blood is drawn from a vein or capillary in the heel, finger, toe, earlobe, back of the hand or inside the elbow. Before the needle is inserted, the puncture site is cleaned with an antiseptic and, if the inside of the elbow is to be used, an elastic band may be placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes blood to fill the veins below the band.

After the needle is inserted, the blood is drawn into an air-tight vial or syringe. The band is removed to restore circulation and the needle is withdrawn. The puncture site will be covered to prevent further bleeding. Enzyme tests typically do not cause much pain for the patient, although a stinging sensation may be felt when the needle is inserted.

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to enzyme tests:

1. Why do you think a liver enzyme test is appropriate for me?
   
   
2. Which type of enzyme test will I undergo?
   
   
3. Will I undergo just one test or several?
   
   
4. Which condition do you suspect might be revealed by this test?
   
   
5. Are there medications that I might be taking that are affecting my enzyme levels?
   
   
6. Are there lifestyle choices that might affect my enzyme levels?
   
   
7. How should I prepare for my enzyme test?
   
   
8. Where on my body will you draw the blood for the enzyme test?
   
   
9. Will I experience any pain or discomfort during the test?
   
   
10. Which treatments are likely to follow my enzyme test?