Clinical Use of Bone Turnover Markers in Osteoporosis

Abstract

The introduction into clinical practice of bone turnover markers (BTM) has made possible the noninvasive research of various metabolic bone diseases. BTM are peptides, produced by bone cells (osteoblasts, osteocytes, and osteoclasts) that are released into the circulation during the process of bone modeling and remodeling. For clinical purposes BTM are divided into markers of bone formation and markers of bone resorption. Bone formation markers reflect osteoblast activity and include osteocalcin, bone specific alkaline phosphatase, N-terminal, and C-terminal propeptide of type I procollagen. Bone resorption markers are products of collagen degradation and include pyridinoline and deoxypyridinolin cross-links, C-terminal telopeptide, N-terminal telopeptide, and tartrate resistant acid phosphatase 5b (TRAP-5b), an enzyme secreted by osteoclasts, reflecting both their number and activity. Although BTM are relatively easily measured in urine and blood samples, there are several drawbacks, such as the relative lack of high specificity for bone and for a specific metabolic bone disease, the inability to exclusively reflect bone formation or resorption but rather bone remodeling or a particular bone compartment (e.g., trabecular vs. cortical bone, endosteal vs. periosteal envelop). Recent data indicate that BTM might have a role in the prediction of fracture risk and monitoring treatment. However, more work is needed concerning the standardization of the measurements, their value in predicting fracture risk independent of BMD and monitoring treatment.