NTx is a marker of bone resorption, based on detecting fragments of type 1 collagen released during osteoclastic resorption and then excreted in urine. Type 1 collagen is formed in bone from the combination of two ?1 and one ?2 collagen polypeptides containing hydroxylated lysine and proline residues. Collagen fibrils are subsequently stabilized by the formation of pyridinium crosslinks, pyridinoline and deoxypyridinoline, in the telopeptide regions. The NTx assay measures N-telopeptide- linked pyridinoline and deoxypyridinoline residues in urine from the breakdown of type 1 collagen. The monoclonal antibody recognises NTx, which contains the cross-linked ?2 chain residues QYDGKGV, a product of osteoclast resorption, but does not recognise non-pyridinoline-linked peptides nor free pyridinoline.
NTx is measured by commercial immunoassay, using either Ostex ELISA or an automated immunoassay (Ortho Clinical Diagnostics) based on the ELISA method. The assay is competitive inhibition EIA using a monoclonal antibody labelled with horseradish peroxidise, and is standardised against collagenase- digested bone collagen. It is reported in units of bone collagen equivalents (nmol)l per mmol urine creatinine (BCE nmol/mmol).

Clinical Use:
Urine NTx can be used to monitor response to antiresorptive therapy, confirm patient compliance and assess efficacy of new therapies in patients with metabolic bone disease such as osteoporosis, Paget’s disease, primary hyperparathyroidism and metastatic bone disease

Patient preparation:
None. A baseline pre-treatment measurement is required if assessing response to antiresorption therapy. Subsequent measurements should be taken at the same time of day.

Comments and interpretation:
Single measurements are of limited value. The least significant change between samples taken at 3 month intervals is +/ 50%. Suppression of NTx by more than 50% from baseline is expected after bisphosphonate therapy as early as 3 months, but routine follow-up may be left to 6 months post therapy.
NTx levels may remain elevated for up to 6-9 months following healing of bone fractures.
Paediatric samples and young adults: Specific age-related reference ranges are not available but NTx levels are significantly higher in children and in young adults.

Sample requirements: Plain urine (no preservative)
Sample volume: 2.5 mL
Specimen Requirements: Ideally a second morning void urine sample
SAS Centres providing NTx:

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