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C Terminal Telopeptide

Bone remodeling allows for bone growth, bone repair and elimination of microfractures. Osteoclasts resorb old bone, while osteoblasts synthesize new protein, known as osteoid. Within several months, osteoid becomes calcified. After the age of 40 years, bone destruction begins to exceed formation, leading to osteoporosis. For every 10% of bone that is lost, the risk of fracture doubles.

The medications most commonly used to treat osteoporosis are estrogen, calcitonin and biphosphonates (etidronate, alendronate, risedronate). Their mechanism of action is to inhibit osteoclastic activity and decrease bone resorption. Treatment with biphosphonates must be continuously monitored because overdosage can eventually weaken bone.

More than 90% of the osteoid matrix of bone consists of type I collagen. Noncollagenous proteins, such as osteocalcin, comprise the remaining 10%. Type 1 collagen is synthesized as procollagen precursor molecules. During bone resorption, osteoclasts secrete proteolytic enzymes that degrade collagen into fragments including C-terminal telopeptide (CTx). As bone ages, the alpha form of aspartic acid present in CTx converts to the beta form. Beta-CTx is released into the bloodstream and can be used as a specific biomarker of bone resorption.

Many diseases are associated with accelerated bone resorption including hyperthyroidism, hyperparathyroidism, osteomalacia, rickets, Pagets disease, multiple myeloma, and bone metastases. Increased bone resorption also occurs during immobilization, glucocorticoid therapy and postmenopausal osteopenia and osteoporosis. Plasma concentrations of Beta-CTx are elevated in these conditions, but most clinical guidelines do not recommend measuring bone markers as part of the diagnostic workup.

Beta-CTx can be used to monitor patients treated with antiresorptive medications such as bisphosphonates or hormone replacement therapy. A baseline level should be measured prior to starting therapy. Beta-CTx should then be measured 3 to 6 months after initiation of therapy. A decrease in beta-CTx concentration of at least 25% from the baseline value indicates an adequate therapeutic response.

Caution must be exerted in interpreting results in patients with compromised renal function, because reduced urinary excretion of beta-CTx results in increased plasma concentrations.

Patient should be fasting prior to blood collection. Specimen requirement is a red top tube of blood. Testing is performed with the Beta-CrossLaps assay. Other names for this test include b-CrossLaps, CTx, C-telopeptide, Beta-Cross-linked, and C-terminal collagen crosslinks.

Expected ranges are: