Biochemical bone metabolism markers in osteoporosis

Potential roles for biochemical bone metabolism markers in osteoporosis include identification of individuals at risk; early diagnosis of bone loss; and determining efficacy of therapy in those with established disease. Bone markers might provide additional and distinct information from bone mass measurement in risk assessment for osteoporosis not only because increased bone turnover occurs after the menopause, but also because this increase persists as long as 40 years after menopause. In normal men, however, only some bone-turnover markers increase slightly with age. Most bone résorption markers measure urinary collagen degradation products and include hydroxyproline, hydroxylysine glycosides, total or free pyridinium type I collagen cross-links and cross-linked N- or C-telopeptides. Serum tartrate resistant-acid phosphatase is an enzyme secreted by osteoclasts. Fasting urinary Ca excretion also measures bone résorption. Bone formation markers are osteoblast products measured in the serum and include alkaline phosphatase, osteocalcin and type IN- or C-Aerminal procollagen propeptides. Measured postmenopausal ultradistal radial bone loss was predicted by a combination of older baseline biochemical markers at menopause. Further longitudinal studies examining other sites and newer bone markers are necessary to confirm this potential role. Markers could also act as early indicators of drug efficacy for alendronate and oestrogen therapy. In evaluating research studies involving bone markers, conclusions depend both on the marker chosen and measurement method. Hie best cross-link to measure remains unresolved. Total deoxypyridinoline and N- or C-telopeptides have advantages because renal clearance of pyridinolines results in free cross-link generation. Bone-specific alkaline phosphatase assays may still cross-react with liver isoenzyme. Both PINP and PICP assays are improving. Assay costs should decrease with automation making them more accessible for use in following individual patient treatment. Currently these assays remain very useful research tests.