Alkaline phosphatase: An enzyme made in the liver, bone, and the placenta and normally present in high concentrations in growing bone and in bile. Alkaline phosphatase is released into the blood during injury and during such normal activities as bone growth and pregnancy. It is measured in a routine blood test.

Abnormally high blood levels of alkaline phosphatase may indicate disease in bone or liver, bile duct obstruction, or certain malignancies. The enzyme is often elevated in the leukemic cells in chronic myelogenous leukemia.

Abnormally low levels of alkaline phosphatase is a genetic condition called hypophosphatasia which results in bone deformities.

The enzyme is termed alkaline phosphatase because it works under alkaline (non-acidic) conditions, as opposed to acid phosphatase.