ALT (Alanine Aminotransferase) – Liver Enzyme Explained
ALT (Alanine Aminotransferase) is a liver enzyme measured in blood tests to assess liver function. Elevated levels may indicate liver damage or disease.
Things worth knowing about "ALT"
ALT (Alanine Aminotransferase) is a liver enzyme measured in blood tests to assess liver function. Elevated levels may indicate liver damage or disease.
What is ALT (Alanine Aminotransferase)?
ALT, also known as Alanine Aminotransferase (ALAT) or formerly as GPT (Glutamate Pyruvate Transaminase), is an enzyme found primarily in liver cells (hepatocytes). It plays a key role in amino acid metabolism. Under normal circumstances, only small amounts of ALT are present in the bloodstream. When liver cells are damaged, ALT is released into the blood, causing its levels to rise. This makes ALT one of the most important and sensitive markers of liver cell injury.
Why is ALT Measured?
ALT is routinely measured as part of blood tests to evaluate liver health. It is considered one of the most specific indicators of liver cell damage. Common reasons for ordering an ALT test include:
- Routine health check-ups and preventive screenings
- Suspected liver disease (e.g., hepatitis, fatty liver)
- Monitoring patients taking potentially liver-toxic medications
- Investigating symptoms such as jaundice, fatigue, or upper abdominal pain
- Follow-up testing for known liver conditions
Normal Values and Reference Ranges
Reference ranges for ALT may vary slightly between laboratories. Generally accepted guidelines are:
- Men: up to approximately 45 U/L (units per liter)
- Women: up to approximately 35 U/L
- Children: varies by age and sex
Values above these thresholds are considered elevated and may indicate liver cell injury.
Causes of Elevated ALT Levels
Elevated ALT levels can have many different causes. The most common include:
Liver Diseases
- Viral hepatitis (Hepatitis A, B, C, D, E): Inflammation of the liver caused by viruses
- Alcoholic liver disease: Liver damage due to excessive alcohol consumption
- Non-alcoholic fatty liver disease (NAFLD): Fat accumulation in the liver, often associated with obesity or diabetes
- Liver cirrhosis: Advanced scarring of liver tissue
- Hepatocellular carcinoma: Primary liver cancer
- Autoimmune hepatitis: Liver inflammation caused by the body's own immune system
Medications and Substances
- Certain pain relievers (e.g., paracetamol/acetaminophen in high doses)
- Statins (cholesterol-lowering medications)
- Antibiotics and antifungal agents
- Herbal supplements and botanical products
Other Causes
- Heart muscle disease (as ALT is also present in cardiac muscle)
- Intense physical exercise
- Thyroid disorders
- Celiac disease (gluten intolerance)
Symptoms Associated with Elevated ALT
An elevated ALT level itself does not cause symptoms -- it is a laboratory finding that may point to an underlying condition. Associated symptoms of liver disease may include:
- Fatigue and general weakness
- Yellowing of the skin and eyes (jaundice / icterus)
- Pressure or pain in the upper right abdomen
- Nausea and loss of appetite
- Dark urine and pale stools
Diagnosis and Further Investigations
An elevated ALT value alone is not sufficient for a diagnosis. A physician will typically order additional tests, including:
- Other liver enzymes: AST (Aspartate Aminotransferase), GGT (Gamma-Glutamyl Transferase), ALP (Alkaline Phosphatase)
- Complete blood count, coagulation values, and bilirubin
- Ultrasound of the liver and bile ducts
- Viral serology (e.g., hepatitis antibody tests)
- Liver biopsy in cases of uncertain findings
Treatment of Elevated ALT Levels
Treatment is always directed at the underlying cause:
- Viral hepatitis: Antiviral medications (e.g., for hepatitis B and C)
- Alcoholic liver disease: Abstinence from alcohol and supportive therapy
- Fatty liver disease: Weight loss, healthy diet, and regular physical activity
- Drug-induced liver injury: Discontinuation of the causative medication
- Autoimmune hepatitis: Immunosuppressive therapy (e.g., corticosteroids)
In many cases, ALT levels return to normal once the underlying cause is addressed.
References
- Schiff E.R., Maddrey W.C., Sorrell M.F.: Schiff's Diseases of the Liver. 12th edition, Wiley-Blackwell, 2018.
- World Health Organization (WHO): Guidelines for the Prevention, Care and Treatment of Persons with Chronic Hepatitis B Infection. WHO, Geneva, 2015. www.who.int
- Kwo P.Y., Cohen S.M., Lim J.K.: ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries. American Journal of Gastroenterology, 2017. doi:10.1038/ajg.2016.517
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