Nucleotidase, 5′ (NTP) is a phosphatase enzyme that acts only on nucleoside 5′ phosphates, such as adenosine 5′ phosphate (AMP; adenylic acid). It is increased 2-6 times in hepatobiliary diseases in which there is intra- or extrahepatic interference with the secretion of bile. In skeletal diseases NTP increases in serum are rarely observed, so assays of NTP are valuable in differentiating between evaluations in alkaline phosphatase caused by hepatobiliary diseases and those caused by diseases of the skeletal system.