Alkaline Phosphatase (ALP)
Also called: Alkaline phosphatase, ALP
ALP is an enzyme on bone-forming and biliary-duct cells. Elevation can mean liver, bone, intestinal or placental origin — GGT helps tie-break.
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What does Alkaline Phosphatase (ALP) measure?
ALP is found on the surface of cells that build bone, line bile ducts and line the small intestine. It is also produced in the placenta during pregnancy. The blood test reports total ALP without specifying source — so an isolated ALP elevation is uninterpretable until you know whether GGT is also elevated (liver origin) or normal (bone, intestinal or placental origin). Children and adolescents have physiologically high ALP from growing bone — not pathological.
What does a high Alkaline Phosphatase (ALP) level mean?
- Cholestasis — bile-duct obstruction (gallstones, tumour, primary biliary cholangitis); GGT also elevated.
- Bone disease — Paget's disease, healing fracture, bone metastases, hyperparathyroidism, osteomalacia; GGT normal.
- Growing children and adolescents — physiological.
- Pregnancy — placental ALP normally rises in third trimester.
- Drug-induced liver injury — some patterns.
What does a low Alkaline Phosphatase (ALP) level mean?
- Zinc deficiency — ALP requires zinc.
- Hypophosphatasia — rare genetic disorder.
- Severe malnutrition, vitamin C deficiency.
- Wilson's disease — uncommonly.
When to discuss with a doctor
Isolated ALP elevation usually warrants checking GGT first to localise the source. Persistent elevation with normal GGT suggests bone disease and warrants further work-up (PTH, calcium, vitamin D, sometimes bone-specific ALP). Persistent elevation with elevated GGT points at liver / biliary disease and warrants ultrasound.