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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.
Levels of ALT may also be markedly elevated (sometimes over 100 times normal) as a result of exposure to drugs or other substances that are toxic to the liver as well as in conditions that cause decreased blood flow (ischemia) to the liver.