N-Telopeptide Cross-links (NTx)

CPT: 82523
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Test Details


  • NTx


Monitor antiresorptive therapy with estrogen and calcium supplementation as well as drug therapy


Enzyme immunoassay (EIA)

Reference Interval

• Male: 5.4−24.2 nM BCE/L

• Female: 6.2−19.0 nM BCE/L

Additional Information

Approximately 90% of the organic matrix of mammalian bone consists of type I collagen that is cross-linked at the N-terminal and C-terminal ends.1 This highly cross-linked structure provides for the basic fabric and tensile strength of bone tissue. The collagen infrastructure of bone undergoes a continuous process of remodeling that involves osteoclast-mediated bone resorption and osteoblast-mediated bone formation. Bone resorption by osteoclasts results in the production of cross-linked N-telopeptides of type I collagen (NTx). NTx is specific to bone and is found in serum and urine as a stable end product of bone degradation. Serum levels of NTx have been shown to correlate well with urine NTx normalized to creatinine.2,3 The NTx test measures the concentration of cross-linked N-telopeptides of type I collagen.1 Levels of NTx correlate with the rate of bone resorption.2,3 Bone resorption rates exceeding bone formation result in a net loss of bone and ultimately osteopenia or osteoporosis.4 Osteoporotic fractures are a major source of morbidity and mortality in older women.5 The level of serum NTx correlates with bone mineral density in elderly women.6 The serum NTx test can be used to monitor the efficacy of antiresorptive therapy with estrogen and calcium supplementation7 as well as drugs such as alendronate.8

Specimen Requirements




0.5 mL

Minimum Volume

0.3 mL (Note: This volume does not allow for repeat testing.)


Red-top tube or gel-barrier tube


Serum must be transferred to a plastic transport tube. Serum must be separated from red cells or gel-barrier immediately after clot formation and centrifugation.

Storage Instructions


Causes for Rejection

Nonserum specimen; serum specimen received on cells or on gel-barrier

Clinical Information

Special Instructions

For monitoring therapy, baseline specimens should be collected just prior to or on the day of therapy initiation. Subsequent specimens for comparison should be collected at approximately the same time of day as the baseline specimen.


1. Endres DB, Rude RK. Mineral and bone metabolism. In: Burtis CA, Ashwood ER, eds. Tietz Textbook of Clinical Chemistry. 3rd ed. Philadelphia, Pa: WB Saunders Co;1999:1349-1457.
2. Clemens JD, Herrick MV, Singer FR, Eyre DR. Evidence that serum NTx (collagen-type I N-telopeptides) can act as an immunochemical marker of bone resorption. Clin Chem. 1997 Nov; 43(11):2058-2063. 9365389
3. Woitge HW, Pecherstorfer M, Li Y, et al. Novel serum markers of bone resorption: Clinical assessment and comparison with established urinary indices. J Bone Miner Res. 1999; 14(5):792-801. 10320528
4. Garnero P, Sornay-Rendu E, Chapuy MC, Delmas PD. Increased bone turnover in late postmenopausal women is a major determinant of osteoporosis. J Bone Miner Res. 1996 Mar; 11(3):337-349. 8852944
5. Prestwood KM, Pilbeam CC, Burleson JA, et al. The short-term effects of conjugated estrogen on bone turnover in older women. J Clin Endocrinol Metab. 1994 Aug; 79(2):366-371. 8045949
6. Scariano JK, Glew RH, Bou-Serhal CE, Clemens JD, Garry PJ, Baumgartner RN. Serum levels of cross-linked N-telopeptides and aminoterminal propeptides of type I collagen indicate low bone mineral density in elderly women. Bone. 1998 Nov; 23(5):471-477. 9823455
7. Prestwood KM, Thompson DL, Kenny AM, Seibel MJ, Pilbeam CC, Raisz, LG. Low dose estrogen and calcium have an additive effect on bone resorption in older women. J Clin Endocrinol Metab. 1999 Jan; 84(1):179-183. 9920080
8. Greenspan SL, Rosen HN, Parker RA. Early changes in serum N-telopeptide and C-telopeptide cross-linked collagen type 1 predict long-term response to alendronate therapy in elderly women. J Clin Endocrinol Metab. 2000Oct; 85(10):3537-3540. 11061497


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
140830 N-Telopeptide, Serum 21215-9 140831 N-Telopeptide, Serum nmol BCE/L 21215-9

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