Alkaline phosphatase (ALP) as a lab test !

Alkaline phosphatase rises when bile flow is blocked or bone turns over fast; GGT tells which source it is.

Updated 2026-11-01

Key facts

Main isoenzymes: hepatobiliary, bone (osteoblasts), placental and intestinal.
High ALP + high GGT = hepatobiliary source (cholestasis); high ALP + normal GGT = bone (or placenta, gut).
It is normally high in growing children and adolescents and in the third trimester of pregnancy.
It is a zinc and magnesium metalloenzyme; a low ALP suggests hypophosphatasia, zinc deficiency or fulminant Wilson disease.
Its reference range depends on age, sex and laboratory.

What it measures: the activity of membrane enzymes that release phosphate from esters at alkaline pH.
Where it comes from: in the liver, the canalicular membrane of hepatocytes and biliary epithelium; in bone, the osteoblasts that mineralise matrix.
Why it rises (liver): biliary obstruction (stone, pancreatic head tumour, cholangiocarcinoma), drug cholestasis, primary biliary cholangitis, sclerosing cholangitis, infiltration (metastases, granulomas).
Why it rises (bone): growth, healing fractures, Paget disease, osteoblastic metastases (prostate), rickets and osteomalacia, hyperparathyroidism.
Pitfalls: blood group O or B secretors raise intestinal ALP after fatty meals; an EDTA sample (a zinc chelator) gives falsely low values.

A very high ALP with normal calcium and phosphate in an older adult suggests Paget disease.
In rickets a high ALP goes with low calcium and phosphate and high PTH.
5′-nucleotidase or isoenzymes help when GGT is not available.

Think of the answer, then tap the question to see it.

How is a hepatic high ALP told apart from a bone one?

With GGT: if it is also high the source is hepatobiliary; if normal, the ALP probably comes from bone.

In which normal states is ALP high?

In growing children and adolescents and in the third trimester of pregnancy (placental isoenzyme).

Which tumour typically raises ALP through osteoblastic bone metastases?

Prostate cancer.

What does an abnormally low ALP suggest?

Hypophosphatasia, zinc or magnesium deficiency, or fulminant Wilson disease.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: Alkaline Phosphatase: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); ALP - blood test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.