Why are ALT and ALP both used in liver disease evaluation, and what patterns help distinguish hepatocellular injury from cholestasis?

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ALT and ALP provide complementary information about liver injury patterns. ALT is concentrated in liver cells and is released in large amounts when hepatocytes are damaged, so it acts as a relatively specific marker of hepatocellular injury. ALP is found in the bile ducts and also in bone, so its elevation can signal cholestasis (bile flow obstruction) or increased bone turnover. Using both together helps distinguish whether liver injury is primarily hepatocellular or cholestatic.

A large rise in ALT with a normal or only modestly elevated ALP points to hepatocellular injury. Conversely, a marked rise in ALP with a relatively smaller increase in ALT suggests cholestasis or biliary obstruction (or a bone-related process, which is why context and sometimes additional tests are useful). In practice, clinicians also consider GGT to confirm a hepatic origin of ALP elevation and to separate bone from liver sources.