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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.

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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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