High Liver Enzymes (ALT, AST): Common Causes and What Happens Next

Your blood test came back with a high ALT or AST, and the word "liver" is now on your mind. You may feel completely well. Mildly high liver enzymes are one of the most common findings on routine blood work. This guide explains what they mean, the usual causes, and what clinicians do next.

What ALT and AST are

Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are enzymes inside liver cells. When liver cells are irritated or damaged, these enzymes leak into the blood [1][2].

• ALT is found mostly in the liver. A high ALT points more directly to the liver.
• AST is found in the liver but also in muscle, the heart, and red blood cells. A high AST alone can come from muscle.

A "liver panel" often also includes [5]:

• Alkaline phosphatase (ALP): linked to the bile ducts and bone.
• Bilirubin: a yellow pigment. High levels cause jaundice (yellow skin or eyes).
• Albumin and INR: show how well the liver makes proteins. These are true measures of liver function.

ALT and AST show irritation. They do not measure how well the liver works.

Common causes

The most common causes of mildly high ALT and AST are [3]:

• Fatty liver linked to metabolism. This is now called metabolic dysfunction-associated steatotic liver disease (MASLD). It was formerly called non-alcoholic fatty liver disease (NAFLD) [4]. It is linked to extra weight around the waist, type 2 diabetes, high blood pressure, and high triglycerides.
• Alcohol use.
• Medicines and supplements. Examples include acetaminophen at high doses, some antibiotics, some seizure medicines, and herbal products.
• Viral hepatitis, especially hepatitis B and C.

Less common causes include hemochromatosis (iron overload), autoimmune hepatitis, celiac disease, thyroid problems, and Wilson disease in younger people.

What your doctor looks at

How high are the numbers?

The American College of Gastroenterology (ACG) guideline suggests that a true healthy upper limit for ALT is lower than many lab ranges: about 29 to 33 U/L for men and 19 to 25 U/L for women [3]. Clinicians often describe the size of the rise compared with the upper limit of normal [3]:

• Mild: under 5 times the upper limit.
• Moderate: 5 to 15 times.
• Severe: over 15 times.
• Very high levels, in the thousands, suggest sudden injury, such as from a virus, a medicine overdose, or poor blood flow to the liver. These need prompt evaluation.

What is the pattern?

• Hepatocellular pattern: ALT and AST are higher than ALP. This points to liver cell injury.
• Cholestatic pattern: ALP and bilirubin are higher. This points to the bile ducts.
• AST to ALT ratio: an AST about twice the ALT or more suggests alcohol-related liver disease [3].

Is it persistent?

A single mild rise often settles. Clinicians usually repeat the test and look at your history first.

The usual next steps

For persistently high levels, the ACG guideline suggests a step-by-step workup [3]:

• A review of alcohol, medicines, supplements, and herbal products.
• Tests for hepatitis B and hepatitis C.
• Iron studies, including ferritin and transferrin saturation, to check for hemochromatosis.
• An abdominal ultrasound to look for fat, blockage, or other changes.
• Further tests for autoimmune or genetic causes when the first tests are normal.

Checking for scarring

For people with fatty liver, the key question is whether there is scarring (fibrosis). Clinicians often use the FIB-4 score, which uses your age, AST, ALT, and platelet count. A low score makes advanced scarring unlikely. A higher score may lead to a liver stiffness scan, such as elastography, or a referral to a liver specialist.

What often helps

For fatty liver, the most important steps usually involve the whole body, not just the liver. Clinicians often discuss gradual weight loss if you have extra weight, regular physical activity, limiting alcohol, and managing blood sugar, blood pressure, and cholesterol. Even a modest weight loss can lower liver fat and bring enzymes down. Your clinician can help you choose goals that fit your health and life.

Canada and the US

Canadian and US clinicians follow a very similar approach. The main differences are in access. Tests like elastography and the timing of specialist referral depend on local health systems. Canada and the US both use the new MASLD name, which was agreed by international liver societies in 2023 [4].

Questions to ask your clinician

• How high are my ALT and AST compared with the upper limit of normal?
• Is the pattern more like liver cell injury or bile duct blockage?
• Could alcohol, a medicine, or a supplement I take be the cause?
• Should I be tested for hepatitis B, hepatitis C, or iron overload?
• Do I need an ultrasound, and what would you look for?
• What is my FIB-4 score, and do I need a liver stiffness test?

When to get care quickly

Go to the emergency department or call 911 if you have:

• Yellow skin or eyes with fever, confusion, or severe tiredness.
• Vomiting blood or black, tarry stools.
• Severe pain in the upper right belly.
• A swollen belly with confusion or drowsiness.
• You took more acetaminophen than the label allows, even if you feel well. Call Poison Control or go to emergency right away.

Bottom line

High ALT and AST are common and usually mild. Fatty liver (MASLD), alcohol, and medicines are the most frequent causes. Clinicians look at how high the numbers are, the pattern, and whether they persist. Then they check for hepatitis, iron overload, and scarring. Most people need a plan to repeat and investigate, not emergency care.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.