What is the normal range of ALT in the blood?

Units per liter. The top of the reference range is set at around 40 u/l for most labs, but this should not be confused with a 'healthy range'. Most folks are well-below, and ethnicity and heredity may affect what is right for a person. The key is to know what to do with a number. The higher a person is above reference range, the more worrisome.