Alanine aminotransferase (ALT), formerly known as SGPT, is a protein found predominantly within liver cells. This enzyme is a standard component of the Liver Function Panel (LFP), a common blood test used to assess liver health. Measuring ALT in the blood provides a specific biomarker for cellular damage, particularly in the liver. An elevated reading suggests that injured liver cells have released their contents into the bloodstream.
The Biological Function of ALT
ALT is an enzyme essential to the body’s metabolism, playing a specialized role in processing amino acids. Its primary function is to catalyze a reaction that transfers an amino group from alanine to alpha-ketoglutarate. This process forms pyruvate and glutamate, which the body uses for energy production and other metabolic pathways.
The enzyme is found in highest concentration inside hepatocytes, or liver cells, but smaller amounts also exist in the kidneys, heart, and skeletal muscles. Under normal conditions, ALT remains contained within these cells, resulting in only low, detectable levels in the serum. When a hepatocyte is damaged due to inflammation or injury, the cell membrane integrity is compromised, allowing the enzyme to leak into the circulation.
Interpreting ALT Test Ranges
The normal reference range for ALT varies depending on the laboratory, testing method, and the patient’s sex and age. The typical range is often cited as 7 to 56 units per liter (U/L) of blood. However, some professional societies suggest the upper limit should be lower, such as 35 U/L for men and 25 U/L for women, as higher values may indicate underlying liver issues.
The clinical importance of an elevated ALT result is determined by the degree of elevation above the upper limit of normal. Mild elevations (up to twice the normal limit) may be caused by temporary factors like vigorous exercise or certain medications. Severe elevations, sometimes reaching levels in the thousands, usually point to acute liver injury. Monitoring the level over time is more informative than a single reading, as ALT levels can fluctuate based on transient factors.
Primary Reasons for Elevated Levels
Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD)
The most frequent cause of persistent ALT elevation is MASLD, formerly known as Non-Alcoholic Fatty Liver Disease (NAFLD). This condition is linked to obesity, type 2 diabetes, and metabolic syndrome, where fat accumulation leads to inflammation and cell damage. In MASLD, the elevation is typically gradual and often remains below 100 U/L.
Medications and Toxins
Elevated ALT cases are also attributable to medications, toxins, and supplements. Over-the-counter pain relievers like acetaminophen, certain statins, and various herbal remedies can cause drug-induced liver injury. The severity varies widely, from mild increases requiring monitoring to acute spikes signaling serious toxicity.
Viral Hepatitis
Viral hepatitis (Hepatitis A, B, and C) is a common cause of elevated ALT. Acute viral infections often result in a sudden release of ALT, with levels spiking 10 to 50 times the normal range. Chronic viral hepatitis, particularly B and C, can cause persistent or mild elevations over many years.
Alcohol and Other Causes
Alcohol consumption is a contributor to liver injury, especially in alcohol-related liver disease. Chronic alcohol use often results in a distinct pattern where the Aspartate Aminotransferase (AST) level is higher than the ALT level, with the AST:ALT ratio exceeding 2. Less common causes include celiac disease, acute heart failure, and muscle injuries like rhabdomyolysis.
Follow-Up Actions and Monitoring
When an initial blood test reveals an elevated ALT level, the first step is a detailed review of the patient’s history, including medication use, diet, and alcohol intake. The healthcare provider will recommend repeating the ALT test within a few weeks to determine if the elevation is transient or persistent. Transient elevations often resolve without further intervention.
If the level remains elevated, additional blood tests are ordered for follow-up evaluation. These include a full liver panel with tests like gamma-glutamyl transferase (GGT), alkaline phosphatase (ALP), and bilirubin to help pinpoint the specific type of liver injury. Screening for viral hepatitis and other underlying metabolic conditions may also be performed.
Lifestyle adjustments are a core part of the management plan, especially for mild elevations linked to MASLD. This includes weight loss through diet and exercise, and the cessation of alcohol or potentially hepatotoxic over-the-counter drugs. Imaging studies, such as an abdominal ultrasound, are used to look for signs of fatty liver disease, structural abnormalities, or biliary obstruction.