Alkaline phosphatase (ALP)

What it measures

ALP is a group of enzymes coming mostly from the liver bile ducts and from bone. It is used to detect cholestasis (blocked bile flow) and conditions with increased bone turnover.

Specimen and preparation

• Specimen: Serum or lithium-heparin plasma
• Tube: Gold top
• Preparation: Preferably fasting; a fatty meal can raise the intestinal fraction in some people. Do not use EDTA or citrate tubes.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Bile duct obstruction (stones, tumour)
• Primary biliary cholangitis or sclerosing cholangitis
• Liver infiltration or metastases
• Paget disease of bone
• Bone metastases, fractures healing, rickets/osteomalacia
• Normal growth and pregnancy

Causes of low values

• Hypophosphatasia
• Zinc or magnesium deficiency
• Wilson disease (fulminant)
• Hypothyroidism
• Sample collected in EDTA

Method

Kinetic colorimetric assay (p-nitrophenyl phosphate, IFCC)

Good to know

A raised GGT alongside ALP points to a liver source; normal GGT suggests bone.

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