Bone ALP (bone-specific alkaline phosphatase, BSAP) is an enzyme produced by osteoblasts — the cells responsible for building new bone. It is released into the bloodstream during the process of bone formation and mineralisation. Because it is derived specifically from osteoblasts rather than other tissues, it provides a direct and specific measure of how actively new bone is being laid down, independent of liver or intestinal ALP.

Bone ALP is one of the key bone formation markers used to assess bone turnover — the continuous process of bone resorption and replacement that maintains skeletal integrity. Elevated bone ALP indicates accelerated bone formation, which occurs in conditions of high bone turnover such as Paget's disease of bone, hyperparathyroidism, bone metastases, and osteomalacia. In osteoporosis management, bone ALP helps track the effectiveness of bone-building medications such as teriparatide.