Alkaline phosphatase (AP)

Alkaline phosphatase (AP, internationally also ALP) is a group of enzymes that occur in different forms: in the liver on the membrane of the bile duct cells, in bone in the bone-forming cells (osteoblasts), and also in the gut and, during pregnancy, in the placenta. In the blood, the shares come mainly from liver and bone. AP is measured in serum; the blood sample is taken in a practice.

What alkaline phosphatase does in the body

AP splits phosphate groups off various molecules and works best in a slightly alkaline environment – hence the name. It needs zinc and magnesium to work. In bone it splits pyrophosphate, which inhibits the deposition of minerals, and at the same time provides phosphate: this allows calcium phosphate to be deposited in the bone matrix, and the bone becomes hard. In the liver it sits on the surface of the bile duct cells and is involved in transport across the cell membrane; in the gut, the local form contributes to detoxifying bacterial components and to fat absorption.

Low alkaline phosphatase – what happens in the body?

If AP activity is low, less pyrophosphate is broken down and the mineralisation of bones and teeth can slow down. Low values occur with zinc or magnesium deficiency, because the enzyme needs these minerals, and also with an underactive thyroid and malnutrition. Rarely, the cause is inherited hypophosphatasia, a change in the ALPL gene: activity then stays permanently low, milk teeth can fall out early, and the bones are softer and break more easily.

Raised alkaline phosphatase – what happens in the body?

A raised AP comes either from the liver or from bone. If bile becomes congested – for example due to gallstones, inflammation of the bile ducts or space-occupying lesions in the liver – the bile duct cells produce more enzyme and release it into the blood; gamma-GT then usually rises too. If bone formation is increased, AP comes from the osteoblasts: quite naturally in growing children and adolescents, and also during the mending of fractures, with vitamin D deficiency and impaired bone mineralisation (osteomalacia) and with Paget's disease. In late pregnancy, AP from the placenta rises.

The interpretation is given in your report.