Different Behaviors of Serum and Urinary CrossLaps ELISA in the Assessment of Bone Resorption in Healthy Girls

Many fragments derived from collagen I degradation come to serum during bone resorption. The characterization of these fragments and the development of assays that can measure them are the basis of most of the biochemical markers of bone resorption. One of these fragments consists of two cross-linked amino acid sequences derived from the C-terminal telopeptide region of type I collagen α1 chains. Fragments containing the sequence EKAH-DGGR (CTX), with a DG site being either nonisomerized (α-CTX) or β-isomerized (β-CTX) were identified. Pyridinoline, deoxypyridinoline, and other cross-links linked both telopeptides (Fig. 1⇓ ) (1). Recent reports have suggested that the proportions of nonisomerized and β-isomerized fragments depend on bone age (1). There are two available assays that can measure the β-isomer form of the CTX epitope: urinary CrossLapsTM ELISA (2), and Serum CrossLaps One Step ELISA (3), both from Osteometer (Denmark). The urinary assay is a polyclonal (rabbit) antibody assay for which the minimal requirement for detection is the presence of …