Alkaline phosphatase (ALP) test: what the result means (ŠF)

Alkaline phosphatase (ALP) is an enzyme produced in the largest amounts by the liver and the bones, and in smaller amounts by the intestine, the kidneys and the placenta during pregnancy. The total ALP measured in the blood reflects the activity of these tissues, which is why the test is used as a first-line marker of liver or bone disorders. In the liver, ALP is most abundant in the cells of the bile ducts, so its level rises particularly when bile flow is disrupted — for example because of stones or narrowing of a duct. In bone, ALP is produced by osteoblasts — the cells that form new bone tissue — so the enzyme rises during the growth period, while a fracture is healing, or in bone diseases. Because a raised ALP can mean either a liver or a bone problem, doctors often order additional tests — GGT, which rises only when there is a liver or bile-duct problem but not with bone causes, or an ALP isoenzyme test — to establish which tissue the change comes from.

• When liver or bile-duct disease is suspected — in the case of jaundice, abdominal pain, itching or darkened urine
• When other liver tests (ALT, AST) show changes — ALP helps to tell whether the problem is related to bile flow
• When bone disease is suspected — for example Paget's disease, bone tumours or bone problems caused by vitamin D deficiency
• To monitor the effect of treatment in liver or bone disease
• As part of a routine biochemical blood test at an annual check-up
• When assessing possible medicine-induced liver damage

A raised alkaline phosphatase is regarded by MedlinePlus as a possible marker of a liver, bile-duct or bone disorder. The most common causes are narrowing of the bile ducts, liver disease or bone disease, and also a healing fracture. Because the enzyme rises for both liver and bone reasons, it needs to be assessed together with other tests.

• Narrowing of the bile ducts (biliary obstruction)
• Liver diseases, including hepatitis
• Bone diseases — Paget's disease, osteomalacia, bone tumours
• A healing bone fracture
• Hyperparathyroidism or hyperthyroidism
• Lymphoma or leukaemia
• Sarcoidosis

What to discuss with a doctor

• GGT and the liver enzymes (ALT, AST) — they help to tell whether the rise comes from the liver or from the bones
• An ALP isoenzyme test, if the source is not clear
• Calcium, phosphorus and vitamin D, if bone disease is suspected
• The medicines taken — some can affect the result, so it is worth discussing them with your doctor
• When to repeat the test so that the change over time can be seen

A low alkaline phosphatase occurs less often than a raised one. According to MedlinePlus, it is linked to zinc or protein deficiency, poor nutrition, pernicious anaemia and thyroid disorders. Always discuss the result with your doctor together with your other tests.

• Zinc deficiency
• Magnesium deficiency
• Protein deficiency or poor nutrition
• Pernicious anaemia
• A thyroid disorder (hypothyroidism)
• Wilson's disease
• Hypophosphatasia (an inherited condition)

What to discuss with a doctor when ALP is low

• An assessment of nutrition — the amount of protein and zinc in the diet
• Thyroid markers (TSH, FT4)
• A complete blood count to rule out pernicious anaemia
• When to repeat the test so that the change over time can be seen

Liver or bone: how the source is established

Doctors often order a GGT test — it rises only when there is a liver or bile-duct problem and does not rise for bone reasons. If GGT is normal while ALP is raised, a bone origin is likely; an ALP isoenzyme test may then be ordered.