Clinical Evaluation of the Elecsys β-CrossLaps Serum Assay, a New Assay for Degradation Products of Type I Collagen C-Telopeptides

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Abstract
Background: The Elecsys β-CrossLaps serum assay measures type I collagen degradation fragments (β-CTx) that contain the β-isomerized octapeptide EKAHD-β-GGR. We investigated the analytical performance of the assay and changes in β-CrossLaps in patients with metabolic bone diseases. Methods: The electrochemiluminescent sandwich immunoassay uses two monoclonal antibodies directed against different regions of the linear EKAHD-β-GGR. Results: β-CrossLaps (β-CTx) immunoreactivity was stable in serum and plasma stored at 4 °C for 24 h or at room temperature for 4 h, and it did not decrease appreciably in samples stored at −30 °C for 12 weeks. Nine cycles of repeated freezing-thawing did not affect serum β-CTx. The intra- and interassay imprecision (CVs) for four samples was ≤2.6% (n = 10) and ≤4.1% (n = 10), respectively. The mean day-to-day biological variation (CV) was 20% in 10 postmenopausal women (n = 10 days). Serum β-CTx and osteocalcin were correlated in patients with hyperparathyroidism (r = 0.796; P r = 0.784; P = 0.0003; n = 16), hypoparathyroidism (r = 0.950; P = 0.0001; n = 11), and pseudohypoparathyroidism (r = 0.987; P = 0.130; n = 4). Serum β-CTx decreased by 47.4% ± 8.8% (mean ± SD) and 60.7% ± 6.5% at 3 and 6 months, respectively, after initiation of estrogen replacement therapy in 34 women. These decreases were greater than the decreases in urinary excretion of deoxypyridinoline (31.8% ± 3.9% and 38.1% ± 4.4%, respectively) or pyridinoline cross-linked C-terminal telopeptide of type I collagen (15.9% ± 3.9% and 16.9% ± 4.6%, respectively). Conclusions: The Elecsys β-CrossLaps serum assay provides a potentially useful tool for assessing bone resorption state, including its response to estrogen replacement therapy.