Labs – ALT and AST
Aspartate Aminotransferase (AST)
- AST is a cytoplasmic enzyme in liver, muscle, RBC’s and other locations
- Released from damaged cells; serum levels peak 24-36 hr after injury and normalize at 3-6 days if injury isn’t ongoing
- Highest elevations occur in viral hepatitis and hepatotoxicity (10-20 times the upper reference limit)
- Mild-to-moderate (3-10 fold) elevations occur in chronic hepatitis, active cirrhosis, chronic passive congestion, drug-induced injury, metastatic tumor, biliary disease and a number of other conditions.
- Cardiac and skeletal muscle injury will also produce substantial AST elevations.
Alanine Aminotransferase (ALT)
- More specific for the liver than AST, but also present in kidney and muscle
- Used to confirm that AST elevations are of liver origin (e.g., elevation of both AST and ALT strongly suggest hepatocellular injury)
- The time course of ALT elevations in liver disease is shown below; AST would show a similar overall time course (though the ratio of the AST and ALT varies with disease and from patient to patient, as described in the next section)
- Viral hepatitis, mononucleosis, and acute hepatotoxicity typically show elevations in ALT that are equal to or greater than AST elevations (AST/ALT less than or equal to 1.0)
- ALT is elevated to a lesser degree than AST in alcoholic liver disease and cirrhosis, passive congestion, bile duct obstruction, or metastatic tumor to the liver (AST/ALT greater than 1.0)
- These are rules of thumb–substantial minorities of patients deviate from them, making the AST/ALT ratio of limited usefulness in the diagnosis of individual patients (though it may be helpful in suggesting the next appropriate step in the workup)