Alanine Aminotransferase (ALT) is an enzyme primarily located within liver cells. Its main function is to help the liver process amino acids, the building blocks of protein, as part of the body’s energy metabolism. When liver cells are healthy, the amount of ALT circulating in the bloodstream remains low. If the liver is damaged or inflamed, the cell walls become permeable, causing the enzyme to leak into the blood in higher concentrations. Measuring the ALT level via a blood test assesses liver health, with an elevated result signaling potential injury to the liver tissue.
Defining ALT and Reference Ranges
ALT is highly concentrated within liver cells. Its presence in the blood indicates that these cells have been damaged or destroyed. The enzyme is integral to energy production in the liver. The half-life of ALT in the circulation is approximately 47 hours before it is cleared.
ALT is measured in units per liter (U/L) of blood serum, though the specific reference range varies by laboratory. A generally accepted range for a healthy adult is approximately 7 to 56 U/L. However, many experts suggest a stricter upper limit of normal, often around 35 U/L for men and 25 U/L for women. This stricter limit acknowledges that even levels within the older “normal” range can signal underlying liver issues.
Elevated ALT interpretations are categorized by magnitude relative to the upper limit of normal (ULN). A mild elevation is up to twice the ULN, while a moderate elevation is defined as two to five times the ULN. Levels exceeding five times the ULN are categorized as marked or severe. Elevations greater than 1,000 U/L are considered massive, often indicating acute liver cell death, but a single high reading can also fluctuate due to non-liver issues like intense physical exercise.
Common Conditions That Elevate ALT
The most frequent cause of elevated ALT is Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD), formerly Non-Alcoholic Fatty Liver Disease (NAFLD). This condition involves the accumulation of excess fat within liver cells, often linked to obesity, insulin resistance, type 2 diabetes, and high cholesterol. The fat accumulation leads to inflammation and injury, releasing ALT into the bloodstream, and may progress to the more severe form, Metabolic Dysfunction-Associated Steatohepatitis (MASH).
Alcohol-related liver injury is another significant cause, ranging from simple fatty liver to alcoholic hepatitis and cirrhosis. Excessive alcohol consumption metabolizes into toxic byproducts that directly damage liver cells. Unlike MASLD, alcohol-related liver disease often causes the aspartate aminotransferase (AST) level to be higher than the ALT level, with an AST-to-ALT ratio greater than two strongly suggesting alcohol as the primary cause.
Certain medications are well-known to cause liver cell injury, resulting in elevated ALT levels. This is known as drug-induced liver injury (DILI) and can be caused by common over-the-counter pain relievers like acetaminophen, especially in high doses. Other prescription drugs, including some statins and certain antibiotics, can also cause DILI, requiring monitoring or a change in medication.
Viral hepatitis, particularly acute infections caused by Hepatitis A, B, and C viruses, is a common source of high ALT levels. Acute viral hepatitis can lead to very high readings, sometimes exceeding 1,000 U/L. Chronic infections, such as long-term Hepatitis B or C, cause more moderate, fluctuating elevations over time. Other causes include autoimmune hepatitis, where the immune system attacks its own liver cells, and celiac disease.
Interpreting Severity and Next Steps
The magnitude of the ALT elevation helps guide medical management. Mild, chronic elevations—those less than twice the ULN—are frequently monitored initially, as they can be transient or caused by non-liver issues. However, an acute elevation greater than 1,000 U/L requires immediate investigation because it indicates a severe event like drug overdose or acute viral infection.
The initial high ALT result is followed by additional blood tests. The AST/ALT ratio is a key diagnostic tool: a ratio greater than 2 to 1 strongly indicates alcohol-related liver damage, while a ratio less than 1 often points toward MASLD. Further tests include Gamma-Glutamyl Transpeptidase (GGT) and Alkaline Phosphatase (ALP) to check for bile duct issues, and bilirubin and albumin levels to measure liver function.
Imaging studies, such as an abdominal ultrasound, are often ordered to visualize the liver and look for signs of fat accumulation (steatosis), scarring (cirrhosis), or blockages in the bile ducts. In cases where the cause remains unclear or advanced scarring is suspected, a liver biopsy may be necessary to examine tissue directly.
Lifestyle modifications are recommended, especially for patients with elevations related to MASLD or alcohol use. Physicians recommend avoiding or significantly limiting alcohol intake, which can reverse liver fat accumulation and inflammation. A balanced diet, focusing on reduced refined sugars and saturated fats, is advised to improve insulin sensitivity and decrease the fat burden on the liver. Regular physical activity, aiming for 150 to 240 minutes of moderate-intensity exercise per week, helps reduce hepatic fat and improve enzyme levels.