More than 90 % of organic bone matrix consists of type I collagen, which is preferentially synthesized in bone. There is regulated anabolism and catabolism of the basic substance in bone. During normal bone metabolism, mature type I collagen is degraded and small fragments pass into the bloodstream and are excreted via the kidneys.

 In physiologically or pathologically elevated bone resorption (e.g. in old age or as a result of osteoporosis), type I collagen is degraded to an increased extent, and there is a commensurate rise in the level of collagen fragments in the blood.

 By determining these bone resorption markers, the activity of osteoclasts can be detected. Especially relevant collagen type I fragments are the β‑isomerized C‑terminal telopeptides (β‑CTx). These isomerized telopeptides are highly specific for the degradation of type I collagen dominant in bone.

Elevated serum levels of isomerized C‑terminal telopeptides of type I collagen have been reported for patients with increased bone resorption. The serum levels return to normal during anti‑resorptive therapy.

Determination of the C‑terminal telopeptides in serum is recommended for monitoring the efficacy of antiresorptive therapy (e.g. bisphosphonates or hormone replacement therapy - HRT) in osteoporosis or other bone diseases. By these means, therapy‑induced changes can be demonstrated after just a few weeks.