The ALT (Alanine Transaminase) test—often called an ALT test—is a common blood test that measures the amount of the enzyme alanine transaminase in your bloodstream. ALT is found mostly in liver cells, so it is considered a key hepatic enzyme and an important marker of liver function and overall hepatic health.

When liver cells are irritated, inflamed, or injured, ALT can leak into the blood. In general, higher ALT levels may suggest liver cell damage (often called liver damage), but the test does not identify the exact cause on its own. Mild ALT increases can occur for many reasons, including certain medications, alcohol use, fatty liver, intense exercise, or recent illness. More significant or persistent elevation may point to a liver disease process that needs further evaluation.

ALT results are typically reported in units per liter (U/L). “Normal” ranges can vary by laboratory, age, and sex, so your result should be interpreted using the reference range listed on your report. Your clinician will also consider trends over time and other labs—especially AST, alkaline phosphatase (ALP), bilirubin, and sometimes GGT—to better understand whether a pattern fits liver inflammation, bile duct problems, or another issue. A normal ALT does not always rule out liver disease, and an elevated ALT does not always mean permanent injury; it’s a signal that the liver may need attention and, if needed, follow-up testing.

The ALT blood test is commonly ordered as part of a liver panel (liver function tests) or a comprehensive metabolic panel. It may be done to screen for liver problems, investigate symptoms, monitor known liver conditions, or check for medication-related liver effects.

Your healthcare provider may order an ALT test if you have symptoms that could suggest liver inflammation or liver disease, such as:

• Fatigue or weakness that doesn’t improve
• Nausea, poor appetite, or unexplained weight loss
• Right upper abdominal discomfort (where the liver sits)
• Jaundice (yellowing of the skin or eyes)
• Dark urine or pale stools
• Itching or generalized malaise
• Easy bruising or swelling (in more advanced liver conditions)

It’s also commonly done if you have risk factors for liver disease or liver damage, including:

• Regular or heavy alcohol use
• Overweight/obesity, insulin resistance, type 2 diabetes, or high triglycerides (risk for fatty liver)
• Possible exposure to viral hepatitis (hepatitis A, B, or C), such as through household contact, sexual exposure, shared needles, or certain travel/food exposures
• Use of medications or supplements that can affect the liver (for example, some cholesterol medicines, certain antibiotics, anti-seizure drugs, high-dose acetaminophen, and some herbal products)
• Known liver disease, prior abnormal liver enzymes, or a family history of liver conditions

ALT is also used to monitor treatment response and disease activity—for example, when managing hepatitis, adjusting medications, or tracking lifestyle changes for fatty liver. Because ALT can rise and fall, repeat testing may be recommended to confirm whether an elevation is temporary or persistent.

• Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH)
• Alcohol-related liver disease
• Viral hepatitis (hepatitis A, B, and C)
• Drug-induced liver injury (including medication- or supplement-related)
• Toxic liver injury (e.g., from alcohol or other toxins)
• Autoimmune hepatitis
• Cirrhosis (advanced scarring of the liver)
• Hemochromatosis (iron overload)
• Wilson disease (copper overload)
• Alpha-1 antitrypsin deficiency

• Monitoring liver function and overall hepatic health over time
• Early detection and follow-up of liver disease risk in people with obesity, diabetes, or metabolic syndrome
• Checking for possible medication-related or supplement-related liver stress (liver enzymes monitoring)
• Supporting lifestyle goals such as reducing alcohol intake, improving nutrition, and tracking changes in liver enzymes with weight management
• General wellness screening when included in routine blood work (especially alongside other liver tests)
• Monitoring recovery after acute illness that can temporarily affect liver enzymes