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Aspartate Aminotransferase (AST)
1. AST is a cytoplasmic enzyme in liver, muscle, RBC's and other locations
2. Released from damaged cells; serum levels peak 24-36 hr after injury
and normalize at 3-6 days if injury isn't ongoing
3. Highest elevations occur in viral hepatitis and hepatotoxicity (10-20
times the upper reference limit)
4. 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.
5. Cardiac and skeletal muscle injury will also produce substantial AST
elevations.
Alanine Aminotransferase (ALT)
1. More specific for the liver than AST, but also present in kidney and
muscle
2. Used to confirm that AST elevations are of liver origin (e.g., elevation
of both AST and ALT strongly suggest hepatocellular injury)
3. 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)
AST/ALT Ratio
1. 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)
2. 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)
3. 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)
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