Measuring the level of alkaline phosphatase (ALP) in blood serum. This enzyme is found in high concentrations in bile duct cells of the liver, bone (osteoblasts), the placenta, and the intestines. It is primarily used as a marker for liver disease (especially biliary obstruction) or bone disease (increased anabolic or catabolic activity). It is often ordered along with other liver enzymes to differentiate between the sources of elevation.