Biochemical markers of bone turnover : clinical usefulness in osteoporosis

The recent development of specific and sensitive biochemical markers reflecting the overall rate of bone formation and bone resorption, has markedly improved the non invasive assessment of bone turnover in various metabolic bone diseases, especially for osteoporosis. The immunoassay of human osteocalcin recognizing the intact molecule and its major proteolytic fragment, assays for bone alkaline phosphatase and the intact form of the N-terminal extension propeptide of type I collagen are currently the most sensitive markers to assess bone formation. The best indices of bone resorption are the new immunoassays for the pyridinoline crosslinks and type I collagen related peptides in urine, but also very recently available in serum. Using these new markers, several studies have shown that bone turnover increases markedly after the menopause and remains elevated in late postmenopausal and elderly women. An increased bone turnover rate is related to a fast rate of bone loss in postmenopausal women and to a decreased bone mass in elderly women. Recent data suggest that some of the new immunoassays for pyridinoline crosslinks and related peptides could predict the subsequent risk of hip fracture in elderly women. Thus, bone markers might be used in combination with bone mass measurement to improve the prognostic assessment of postmenopausal women, i.e. their risk of developing osteoporosis and ultimately fractures. Treatment of postmenopausal women with antiresorptive drugs such as estrogens, bisphosphonates and calcitonin is followed by rapid decrease of the levels of bone markers that is correlated with the long term increase of bone mass as assessed by dual-energy X ray absorptiometry measurement. Thus, bone markers should be useful in monitoring treatment efficacy in patients with osteoporosis. Appropriate combination of the most efficient markers of bone formation and resorption will probably provide a powerful tool in the clinical investigation of osteoporosis.