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Alanine Aminotransferase Levels: What's Normal?

Marshall M. Kaplan, MD
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Tufts–New England Medical Center; Boston, MA 02111

Requests for Single Reprints: Marshall M. Kaplan, MD, Tufts–New England Medical Center, 750 Washington Street, Box 233, Boston, MA 02111; e-mail, mkaplan@lifespan.org.

Ann Intern Med. 2002;137(1):49-51. doi:10.7326/0003-4819-137-1-200207020-00012
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Alanine aminotransferase (ALT), a sensitive indicator of liver cell injury, has been used to identify patients with liver disease for almost 50 years (1). This cytosolic enzyme, which is found in many organs, catalyzes the transfer of the α-amino group from alanine to α-ketoglutaric acid (2). Alanine aminotransferase levels are particularly high in the liver. For detection of liver diseases, ALT is thought to be a more specific indicator than aspartate aminotransferase, an enzyme found in cytosol and mitochondria (3). Serum levels of ALT normally are low—167 to 667 nkat/L (10 to 40 U/L) in most laboratories; however, normal values may vary greatly among laboratories. Elevated serum ALT levels help identify many types of liver diseases in patients and were widely used in the 1980s to screen blood donors for non-A, non-B hepatitis. Any type of liver cell injury can modestly increase ALT levels. Values up to 5000 nkat/L (300 U/L) are considered nonspecific. Marked elevations—that is, ALT levels greater than 8350 nkat/L (500 U/L)—occur most often in persons with diseases that affect primarily hepatocytes, such as viral hepatitis, ischemic liver injury (shock liver), and toxin-induced liver damage. Despite the association between greatly elevated ALT levels and hepatocellular diseases, the absolute height of the ALT elevation does not correlate with the extent of liver cell damage (4). Accordingly, the absolute ALT elevation is of little prognostic value. Currently, measurement of serum ALT levels is the most frequently used test to identify patients with liver diseases.

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