C-TERMINAL TELOPEPTIDE, β-CTX, β-CROSSLAPS

• Clinical Applications

  The assay is specific for an octapeptide in the C-terminus of the α₁ chain of type 1 collagen and accurately reflects osteoclast-mediated bone resorption. There is a significant diurnal variation (peak serum CTX between 2-6am) which is blunted after fasting. Samples should thus optimally be collected fasting in the early morning and specimen collection should be consistent during monitoring visits.

  Clinical applications include

  1. Assessing bone turnover (osteoporosis, hyperparathyroidism, Paget's disease, thyrotoxicosis, immobility).
  2. Evaluating the efficacy of anti-resorptive therapy - there is a mean 70% fall of β-CTX from baseline 3-6 months after initiating biphosphonate therapy and 50% after oestrogen replacement. Preliminary data suggests that a decline of >40% in β-CTX at 6 months has a 90-95% positive predictive value that a decline in bone density will be prevented on repeat DEXA scan 2 years later.
  3. Prospective studies demonstrate that levels of bone resorption markers (including β-CTX) are associated with increased rate of bone loss and fracture risk independently of bone density. However, the translation of these findings into clinical practice requires further study.
  
  
• Interpretation

  Increased levels of β-CTX occur in osteoporosis, Paget's disease, primary hyperparathyroidism, renal insufficiency and bone metastases.

  Due to the diurnal rhythm of β-CTX it is recommended that samples be taken fasting morning (0800-0900am) or mid-afternoon (0200-0400pm)