Alkaline phosphatase (ALP) is an enzyme found in multiple tissues including the liver, bile ducts, bone, kidneys, and intestines. In routine blood testing, an elevated ALP most commonly indicates liver or bile duct disease, or bone disorders — as these are the two most significant sources of ALP in adults.

ALP rises when there is bile duct obstruction, cholestasis (impaired bile flow), liver infiltration, or bone conditions involving increased bone turnover such as Paget's disease, bone metastases, or hyperparathyroidism. Because ALP can come from multiple sources, an elevated result is typically followed by ALP isoenzyme testing or GGT measurement to identify the organ of origin.