AST blood test: what aspartate aminotransferase measures, normal levels, and what high AST means

AST is an enzyme that helps cells process amino acids, and it is present in many tissues with high energy demands: liver, skeletal muscle, heart muscle, kidney, brain and red blood cells. When cells in any of these are injured, AST leaks into the blood. That breadth is the enzyme's strength and its weakness. It rises in liver disease, but it also rises after a hard workout, a muscle injury, a heart attack or hemolysis, so a high AST is interpreted alongside ALT, which is nearly liver-specific, and creatine kinase, which is muscle-specific.

Like ALT, AST reports injury, not function. The liver's actual working capacity is judged from albumin, bilirubin and the clotting time. AST clears from the blood a little faster than ALT, which is one reason the ratio between them changes with the type and duration of injury.

AST is on every comprehensive metabolic panel and hepatic panel, so it is checked routinely. It is ordered deliberately in the same situations as ALT: risk factors for fatty liver disease, alcohol use above moderate levels, medications that can injure the liver, viral hepatitis, and symptoms such as fatigue, right upper abdominal discomfort, dark urine or jaundice. It is also useful when muscle injury is a possibility, in statin-related muscle symptoms, after extreme exercise, and in suspected rhabdomyolysis, where it rises alongside creatine kinase.

AST 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. Ordering the hepatic panel gives ALT, alkaline phosphatase, bilirubin and albumin for the same draw; adding GGT helps separate alcohol from other causes, and adding creatine kinase settles whether muscle is the source.

Avoid strenuous exercise for 48 to 72 hours before the draw; intense or unaccustomed exercise, especially weight training, raises AST for several days, and a very high value after a hard session is muscle, not liver. Avoid alcohol for at least two to three days. Fasting is required for the glucose on a metabolic panel. List all medications and supplements, and mention recent illnesses, injuries or injections into muscle.

Most laboratories report a normal AST of about 8 to 48 units per liter for men and 8 to 43 for women; some use a single range to around 40, and the range on your report applies. As with ALT, values just above the range are common and are read in context.

The ratio of AST to ALT carries information. In fatty liver disease, viral hepatitis and most medication injuries, ALT is higher than AST. An AST at least twice the ALT is the classic pattern of alcohol-related liver disease and is also seen as scarring advances toward cirrhosis from any cause; a rising ratio over time in someone with known fatty liver is one signal that the disease is progressing, and the FIB-4 score uses the two enzymes with age and platelets to estimate that risk. An isolated high AST with a normal ALT usually means the source is not the liver at all: muscle, red cells or, rarely, a benign laboratory variant called macro-AST.

When AST and ALT are both high with ALT higher, the causes are those of liver cell injury: fatty liver disease, medications and supplements, viral hepatitis B and C, autoimmune hepatitis, hemochromatosis and the rarer inherited conditions. When AST is higher than ALT, alcohol moves to the top of the list, along with advanced scarring from any cause. When AST is high and ALT is normal, the liver is usually not the source: strenuous exercise, muscle injury or inflammation, statin-related myopathy, seizures, intramuscular injections, hemolysis and, historically, heart attack, all raise AST from other tissues, and creatine kinase and LDH point to which.

The size of the elevation matters. Values under two to three times the upper limit are the common case and are evaluated over weeks; values in the hundreds suggest acute hepatitis or significant muscle injury; values in the thousands indicate acute severe injury from acetaminophen, shock, an acute viral infection or rhabdomyolysis and are an emergency, particularly with jaundice, confusion or dark urine.

A low AST has no clinical significance. It is seen in vitamin B6 deficiency, kidney failure and in people with low muscle mass, and requires no follow-up.

Repeat in two to four weeks after 48 to 72 hours without hard exercise and several days without alcohol, with creatine kinase on the same draw if AST was the higher enzyme. If the elevation is confirmed and creatine kinase is normal, the liver evaluation follows: hepatitis B and C testing, ferritin and transferrin saturation, GGT, a full hepatic panel, a metabolic assessment and a liver ultrasound, then autoimmune, celiac, thyroid and inherited-disease testing if those are unrevealing. If creatine kinase is high, the question becomes the muscle: recent exercise, statins and other drugs, thyroid function, and a clinical review for an inflammatory muscle disease.