What is Alanine aminotransferase (ALT) or GPT?

ALT, also called GPT, is a transaminase-type enzyme found mostly in the liver, although it is also present in other organs such as the kidneys, heart, muscles and pancreas.
This enzyme is primarily measured to assess the health of the liver. When the liver is damaged in any way, it releases ALT, along with other transaminases, into the bloodstream, causing an increase in detectable levels on blood tests.
The ALT test is usually performed together with other parameters that help determine the status of the liver, such as AST (aspartate aminotransferase), alkaline phosphatase, lactate dehydrogenase (LDH) and bilirubin. When elevated ALT and AST values are observed, liver cell damage, i.e. hepatocellular pathology, is usually suspected.

Why is this analysis important?

• Detect liver diseases such as cirrhosis and hepatitis.
• Determine whether jaundice, which manifests as yellowing of the skin and eyes due to increased bilirubin, is caused by liver problems or other causes.
• Monitor possible liver damage caused by some medications.
• Assess liver damage that may be associated with intense physical exercise or overtraining.

At Ailin we offer you health tests that include this marker, such as the essential blood test and the sports performance test. Both measure both ALT and AST, another biomarker studied in combination to estimate liver damage.

What are the normal ALT values?

ALT reference values may vary between laboratories, so the physician interprets results based on the clinical context and other individual factors. According to the American Association for the Study of Liver Diseases (AASLD), values above 30 U/L should be considered abnormal.

Levels of ALT may be high in different circumstances:

• Paracetamol toxicity.
• Liver injury (chronic such as cirrhosis, or acute such as hepatitis).
• Excessive alcohol consumption.
• Growth spurts, especially in children and adolescents.
• Excessive exercise or muscle damage.
• Mononucleosis and other viral infections.
• Exposure to carbon tetrachloride or other toxins.
• Some drugs such as statins, antibiotics or opioids.
• Other causes.

The low levels ALT levels are less common and are generally not as much of a clinical concern as elevated levels. However, some reasons why ALT may be low include:

• Vitamin B6 (pyridoxine) deficiency.
• Malnutrition.
• Ageing.
• Significant loss of muscle mass.
• Pregnancy.

Ratio between ALT and AST (Aspartate aminotransferase)

It is common for doctors to compare ALT and AST values to calculate the AST/ALT ratio, which can help identify whether liver damage is related to alcohol consumption. However, this ratio can also be altered in other liver diseases, such as non-alcoholic steatohepatitis.

Although increased ALT levels in the blood can have many causes, further testing is always necessary to identify the exact source of the problem.

Bibliographical references 

Rinella ME, Neuschwander-Tetri BA, Siddiqui MS, Abdelmalek MF, Caldwell S, Barb D, Kleiner DE, Loomba R. AASLD Practice Guidance on the clinical assessment and management of nonalcoholic fatty liver disease. Hepatology. 2023 May 1;77(5):1797-1835. doi: 10.1097/HEP.0000000000000323. Epub 2023 Mar 17. PMID: 36727674; PMCID: PMC10735173.

Liu Z, Que S, Xu J, Peng T. Alanine aminotransferase-old biomarker and new concept: a review. Int J Med Sci. 2014 Jun 26;11(9):925-35. doi: 10.7150/ijms.8951. PMID: 25013373; PMCID: PMC4081315.

Katarey D, Verma S. Drug-induced liver injury. Clin Med (Lond). 2016 Dec;16(Suppl 6):s104-s109. doi: 10.7861/clinmedicine.16-6-s104. PMID: 27956449; PMCID: PMC6329561.