Bone alkaline phosphatase (BALP) test: what the result means

Alkaline phosphatase (ALP) is an enzyme found throughout the body, but the largest amounts of it are produced by the liver, the bones, the intestine and the placenta. Because the total ALP test does not show which tissue the enzyme came from, an isoenzyme (fraction) test is carried out when the result is raised — it makes it possible to separate the bone fraction (BALP) from the liver fraction. Bone alkaline phosphatase is produced by osteoblasts — the cells that form new bone tissue — so the amount of BALP reflects how intensive bone turnover (formation) is. The test is useful when total ALP is raised and it needs to be established whether the cause lies in the bones or in the liver, and also when assessing Paget's disease, bone tumours or osteomalacia. According to MedlinePlus, the normal range of total alkaline phosphatase in adults is about 40–129 IU/l, but the reference limits of the isoenzymes (BALP among them) differ between laboratories and are not separately standardised.

• When the total alkaline phosphatase (ALP) test shows a raised value and it needs to be established whether the cause of the rise lies in the bones or in the liver
• When Paget's disease or another disorder of bone turnover is suspected
• When assessing osteoblastic bone tumours or bone metastases
• When monitoring the healing of a fracture or osteomalacia (softening of the bones)
• When assessing the effect of hyperparathyroidism on the bones

A raised bone alkaline phosphatase shows increased osteoblast activity — this may be a healing fracture, Paget's disease, osteoblastic bone tumours, osteomalacia or hyperparathyroidism. Total ALP can also rise for liver reasons, which is why an isoenzyme test is carried out, to establish which tissue the rise came from.

• A healing bone fracture
• Paget's disease (a disorder of bone turnover)
• Osteoblastic bone tumours
• Osteomalacia (softening of the bones)
• Hyperparathyroidism

Symptoms that can accompany a raised result:

• Bone pain
• Joint pain
• Irregular bone shapes
• Fractures

What to discuss with a doctor when the result is raised:

• A total ALP and isoenzyme (fraction) test, to clarify whether the rise is of bone or of liver origin
• Calcium, phosphorus, vitamin D and parathyroid hormone (PTH) tests
• Bone imaging tests, if Paget's disease or a tumour is suspected
• When to repeat the test, so that the change over time can be seen

A low bone alkaline phosphatase shows slowed osteoblast activity. Possible causes are the rare genetic disease hypophosphatasia, hypothyroidism, protein or zinc deficiency and poor nutrition. A low total ALP is always worth discussing with a doctor, because the range of possible causes is wide.

• Hypophosphatasia — a rare inherited bone disease
• Hypothyroidism (reduced thyroid function)
• Protein deficiency or poor nutrition
• Zinc deficiency

What to discuss with a doctor when the result is low:

• A total ALP and isoenzyme test, to clarify the cause
• A thyroid function (TSH) test
• An assessment of nutrition
• Genetic counselling, if hypophosphatasia is suspected

Related bone tests

Bone alkaline phosphatase is one of the bone markers. Related indicators on this site: alkaline phosphatase, osteocalcin, beta-CrossLaps, TRAP, calcium and vitamin D.