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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# 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.

## In detail

- 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.

**Also called**
ALP, Serum alkaline phosphatase

## Clinical notes

- 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.

## Related conditions

- [Choledocholithiasis](https://durgical.com/en/conditions/choledocholithiasis/)
- [Rickets and osteomalacia](https://durgical.com/en/conditions/rickets-and-osteomalacia/)
- [Primary hyperparathyroidism](https://durgical.com/en/conditions/primary-hyperparathyroidism/)
- [Wilson disease](https://durgical.com/en/conditions/wilson-disease/)
- [Pancreatic cancer](https://durgical.com/en/conditions/pancreatic-cancer/)

## Related

- [Gamma-glutamyl transferase (GGT) as a lab test](https://durgical.com/en/biochemistry/topics/gamma-glutamyl-transferase-as-a-lab-test/)
- [Liver function tests: reading the pattern](https://durgical.com/en/biochemistry/topics/liver-function-tests-reading-the-pattern/)
- [Calcium and phosphate](https://durgical.com/en/biochemistry/topics/calcium-and-phosphate/)
- [Vitamin D (calcitriol)](https://durgical.com/en/biochemistry/molecules/vitamin-d/)
- [Zinc](https://durgical.com/en/biochemistry/topics/zinc/)

## Test yourself

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.

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

Sources: [Alkaline Phosphatase: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/alkaline-phosphatase/) (MedlinePlus, US National Library of Medicine, 2026); [ALP - blood test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003470.htm) (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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