Aspartate Aminotransferase (AST) is an enzyme which exists in very high levels within highly metabolic tissue. This includes liver cells, tissues of the heart, and skeletal muscle cells. Aspartate Aminotransferase is found in a lower concentrations in Red Blood Cells, and tissues of the kidneys and pancreas. When disease or injury affects the cells of these tissues, the cells lyse. The AST is released, picked up by the blood, and the serum level rises. The amount of AST elevation is directly related to the number of cells affected by the disease or injury. Furthermore, the elevation depends on the length of time that the blood is drawn after the injury. AST is cleared from the blood in a few days. Serum AST levels become elevated 8 hours after cell injury, peak at 24 to 36 hours, and return to normal in 3 to 7 days. If the cellular injury is chronic, levels will be persistently elevated.
Because AST exists within the liver cells, diseases that affect the hepatocyte will cause elevated levels of this enzyme. In acute hepatitis, AST concentration may increase 20 times the normal levels. In acute extrahepatic obstruction (e.g., gallstone), AST concentrations can quickly elevate to 10 times the normal values then decrease quickly. In patients with cirrhosis, the level of AST depends on the amount of active inflammation.
Serum levels of Aspartate Aminotransferase are usually compared to Alanine Aminotransferase (ALT) levels in terms of AST/ALT Ratio. Patients who have Alcoholic Cirrhosis, Liver Congestion, on Metastatic Tumor of the Liver usually have higher Aspartate Aminotransferase levels than Alanine Aminotransferase levels (AST /ALT ratio is greater than 1). On the other hand, patients with Acute Hepatitis, Viral Hepatitis, or Infectious Mononucleosis usually have lower Aspartate Aminotransferase levels than Alanine Aminotransferase levels (AST/ALT ratio is less than 1). In situations when AST levels are 10 times or more than normal values, an AST/ALT ratio less than 1 won’t be accurate to use.
Patients with acute pancreatitis, acute renal diseases, musculoskeletal diseases, or trauma may have a transient rise in serum AST. Patients with RBC abnormalities such as acute hemolytic anemia and severe burns also can have elevations of this enzyme. AST levels may be decreased in patients with beriberi or diabetic ketoacidosis and in patients who are pregnant.
Causes of False Aspartate Aminotransferase Indications
- Pregnancy may cause decreased AST levels.
- Exercise may cause increased levels.
- Levels of Aspartate Aminotransferase are falsely decreased in patients with pyridoxine deficiency (beriberi, pregnancy), severe long-standing liver disease, uremia, or diabetic ketoacidosis.
- Drugs that may cause increased levels include Antihypertensives, Cholinergic Cgents, Coumarin-type Anticoagulants, Digitalis Preparations, Erythromycin, Hepatotoxic medications, Isoniazid, Methyldopa, Oral Contraceptives, Opiates, Salicylates, Stains, and Verapamil.
- Intramuscular Injections may affect the test results if given to the patient before the Aspartate Aminotransferase test.
Normal Aspartate Aminotransferase Levels
Newborns have the highest Aspartate Aminotransferase levels. Aspartate Aminotransferase levels normally decrease as the person grows in age. Females tend to have slightly lower levels than males. The following is a listing of normal Aspartate Aminotransferase levels for different age groups:
Newborn upt to 5 days of age: 35 to 140 units/L.
Children less than 3 years: 15 to 60 units/L..
Children between 3 and 6 years: 15 to 50 units/L.
6 to 12 years : 10 to 50 units/L.
12 to 18 year: 10-40 units/L.
Adults: 0-35 units/L
Causes of High Aspartate Aminotransferase Levels
When liver cell injury occurs. The cells die and lysis of the cell occurs. The contents of the cell (including AST) will be spewed out and are collected into the blood. Elevated AST levels thereby occur. Disease that cause liver cell injuries inclue:
- Hepatic Cirrhosis.
- Drug-induced Liver Injury.
- Hepatic Metastasis.
- Hepatic Necrosis: High AST levels are detected only in initial stages of Hepatic Necrosis.
- Hepatic Surgery.
- Infectious Mononucleosis with Hepatitis.
- Hepatic infiltrative process (e.g., tumor).
When muscle cell injury occurs, the cells die and lysis of the cell occurs. As with liver cell injuries, the contents of the muscle cell including AST are spewed out and are collected into the blood causing high AST levels to occur. Conditions that cause muscle cell injury include:
- Skeletal Muscle Diseases.
- Skeletal muscle trauma.
- Recent Noncardiac Surgery.
- Multiple Traumas.
- Severe, Deep Burns.
- Progressive Muscular Dystrophy.
- Recent Convulsions.
- Heat Stroke.
- Primary Muscle Diseases (e.g., Myopathy, Myositis).
Other diseases that cause cell injury will cause the cells die and lysis of the cell occurs. As a result the contents of the cell (including AST) will be spewed out and are collected into the blood which leads to high AST levels to occur. This occurs when patients have Acute Hemolytic Anemia or Acute Pancreatitis.
Causes of Low Aspartate Aminotransferase Levels
- Acute Renal Disease.
- Diabetic Ketoacidosis.
- Chronic Renal Dialysis.