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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.
The ALT blood test measures the level of the enzyme alanine transaminase (ALT) in the blood. ALT is found primarily in the liver, but is also found in smaller amounts in the kidneys, heart, muscles, and pancreas.