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Labs determine 'normal range' by surveying the people using that lab. For my lab, normal ALT is 6-34 and normal AST is 9-34. ALT is made in the liver so high levels indicate it is being leaked out of damaged liver cells into the bloodstream.
An alanine aminotransferase (ALT) test measures the level of ALT enzyme in your blood. ALT is an enzyme produced by liver cells. It helps the body metabolize proteins. If the liver is damaged or not functioning properly, ALT is released into the blood. This causes ALT levels to increase.