ALT is the intracellular enzyme that is present in many cells of human body, but its concentration is especially high in liver cells and lower in cardiac muscle. This substance plays the role of catalyzer of complex chemical reactions but its function is not that significant for doctors. Its quantity in the blood is what is clinically significant. As was mentioned above ALT is contained within cells, therefore elevation of its level in the blood points to the destruction of the very cells. Since the ALT content is mostly in the liver then clinical interpretation is aimed for precise revealing of hepatic disorders. Due to its lower cost the given test may be referred to screening, that is available for widespread use. Diagnosing based on elevated ALT only is not allowed. Doctor should take into account complaints, symptoms and complex laboratorial investigations. ALT level can exceed normal range not only due to some diseases. It can be connected with intensive physical load on the eve or with choline deficiency in the food. This test is rarely prescribed separately as maximum information value is available during testing ALT, AST and alkaline phosphatase. If AST level is higher than ALT then it is far more likely that problem is related to the heart. And if alkaline phosphatase level is elevated then most likely it’s related to the biliary disease and not the liver itself. Tests should be taken in one laboratory and in accordance with norms that are given there in order to control treatment. Since the same marker can be differently interpreted in different medical organizations.