ALT blood test: what alanine aminotransferase measures, normal levels, and what high ALT means

ALT is an enzyme that liver cells use to process amino acids. It sits inside the cells, so the blood normally carries only a small amount; when liver cells are damaged or die, ALT spills out and the blood level rises within hours. Because the liver holds far more ALT than any other organ, a high ALT points at the liver in a way that AST, which is also abundant in muscle, heart and red cells, does not.

ALT tells you that liver cells are being injured and roughly how much; it does not say why, and it does not measure how well the liver is working. Function is judged from albumin, bilirubin and the clotting time (INR), which is why a liver panel includes them. A person can have cirrhosis with a normal ALT once few cells remain to leak, and a healthy person can have a transient spike from a workout or a medication.

The result is reported in units per liter. Two upper limits are in use: the ranges printed by most laboratories, which reach the mid-40s to mid-50s, and the stricter limits recommended by the American College of Gastroenterology, about 29 to 33 for men and 19 to 25 for women, derived from populations screened for liver disease. Values between the two are common and are read in context rather than dismissed.

ALT is on every comprehensive metabolic panel and hepatic panel, so most adults have it checked at least yearly. It is ordered deliberately for people with risk factors for fatty liver disease, including obesity, type 2 diabetes, high triglycerides and metabolic syndrome; for anyone drinking above moderate levels; before and during treatment with medications that can injure the liver, such as statins, methotrexate, isoniazid, antifungals and some antibiotics and antiseizure drugs; for people with viral hepatitis or at risk of it; and for symptoms such as fatigue, right upper abdominal discomfort, dark urine, pale stools, itching or yellowing of the eyes.

Anyone using anabolic steroids, high-dose supplements known to affect the liver, including green tea extract, kava and some bodybuilding and weight-loss products, or injectable peptides should have ALT checked, because these are a growing cause of unexplained elevations.

ALT comes with the comprehensive metabolic panel and the hepatic function panel, and can be ordered alone, from your doctor or online through TestWell for a draw at a Quest or Labcorp patient service center, with a licensed physician reviewing the order and results in your portal within one to three business days. The hepatic panel adds AST, alkaline phosphatase, bilirubin and albumin and is the better order when the liver is the question; GGT in the same draw helps separate alcohol and bile-duct causes.

Fasting is required for the glucose on a metabolic panel and is reasonable for a liver panel. Avoid strenuous exercise for 48 hours before the draw, because muscle work raises ALT and especially AST, and avoid alcohol for at least two to three days, since a recent heavy intake raises both. Take prescribed medications as usual and list them, with every supplement, for the ordering physician; a recent viral illness, including a cold, can raise ALT for a few weeks and is worth mentioning.

Most laboratories report a normal ALT of about 7 to 55 units per liter for men and 7 to 45 for women; some use a single range to around 40 or 50, and the range on your report applies. The stricter healthy-population limits described above, roughly 33 for men and 25 for women, are what liver specialists increasingly use to decide who needs evaluation, particularly in people with metabolic risk factors.

ALT is read with AST and the ratio between them. ALT higher than AST is the usual pattern of fatty liver disease, viral hepatitis and most medication injuries. AST higher than ALT, especially by two to one or more, points toward alcohol, advanced scarring, or a muscle source; when AST alone is high, creatine kinase identifies muscle. Both enzymes in the hundreds suggest acute hepatitis from a virus, a drug or a toxin; in the thousands, acute severe injury from acetaminophen overdose, shock or an acute viral infection, which is an emergency. A mild elevation, under about two to three times the upper limit, is the common case and is worked up over weeks.

The most common cause of a mildly high ALT in the United States is metabolic dysfunction-associated steatotic liver disease, the fat accumulation in the liver that accompanies excess weight, insulin resistance, type 2 diabetes and high triglycerides; it affects roughly a third of adults and is usually silent. Alcohol is next, with a threshold that varies between people, followed by medications and supplements: statins (usually mild and rarely a reason to stop), acetaminophen at high doses, antibiotics such as amoxicillin-clavulanate, antifungals, methotrexate, isoniazid, some antiseizure drugs, anabolic steroids, and herbal products.

Viral hepatitis B and C are the causes most important not to miss, because both are treatable and both are often silent; a one-time hepatitis C test is recommended for every adult. Less common causes include hemochromatosis (iron overload), autoimmune hepatitis, celiac disease, thyroid disease, alpha-1 antitrypsin deficiency, Wilson's disease in younger people, muscle injury, and heart failure. A brief elevation after a viral illness or a hard workout resolves on its own.

Symptoms rarely accompany a mildly high ALT; when they do, fatigue, nausea, right upper abdominal discomfort, dark urine, itching or yellow eyes point to more significant injury. A high ALT with jaundice, confusion or easy bruising is urgent.

A low ALT has no clinical significance. Very low values are seen in older adults, in people with low muscle mass or frailty and in vitamin B6 deficiency, and population studies associate them with frailty rather than with any liver problem. No follow-up is needed for an ALT below the reference range.

A first mildly high ALT is repeated in two to four weeks after avoiding alcohol and hard exercise and reviewing medications and supplements. If it stays high, the standard first-line evaluation is hepatitis B and C testing, iron studies with ferritin and transferrin saturation, a metabolic assessment (glucose or A1c, lipids, weight and waist), a full hepatic panel with GGT, and a liver ultrasound to look for fat and structure. Autoimmune markers, celiac testing, thyroid function, ceruloplasmin in younger people and alpha-1 antitrypsin follow if those are unrevealing. Persistently elevated enzymes over six months, or any elevation over about five times the upper limit, warrant a specialist review; fatty liver disease is staged with a FIB-4 score calculated from age, AST, ALT and platelets, and sometimes with elastography.