Bone-specific alkaline phosphatase

Ranges depend on the assay and are printed separately for men, premenopausal and postmenopausal women by some labs; it is much higher in children and teenagers.

Bone-specific alkaline phosphatase is the fraction of the alkaline phosphatase enzyme that comes from the cells that build new bone, as opposed to the fraction from the liver. It is a marker of how actively bone is being formed. The test measures this bone fraction in a blood sample, usually by an immunoassay that reports in micrograms per litre.

Doctors request it when a raised total alkaline phosphatase needs to be traced to bone or liver, and in bone conditions such as Paget's disease, thinning of the bones and the bone changes of long-term kidney disease, where it is used to follow the effect of medicines over time.

• Growth in childhood and adolescence
• Paget's disease of bone
• Low vitamin D and softening of the bones
• Healing fractures
• Bone changes of long-term kidney disease
• Overactive parathyroid or thyroid glands
• Cancer that has spread to bone

• Bisphosphonate and other medicines that slow bone turnover
• Underactive thyroid
• Some rare inherited conditions of bone
• Long-term steroid tablets

• A healing fracture raises it for weeks
• The bone and liver forms overlap in most assays, so a raised liver ALP lifts this reading slightly
• Different assays and units (ug/L versus U/L) are not interchangeable

A morning sample is preferred as bone turnover follows a daily rhythm. Best followed within one lab, as assays differ.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.