Abstract
Bone mineral density (BMD) can be measured by a variety of techniques at several skeletal sites. Once measured, the manufacturers’ software uses the BMD to calculate a T-score and/or Z-score. Both T-scores and Z-scores are derived by comparison to a reference population on a standard deviation scale. The recommended reference group for the T-score is a young gender-matched population at peak bone mass, while the Z-score should be derived from an age-matched reference population. T-scores and Z-scores are widely quoted in scientific publications on osteoporosis and BMD studies, and are the values used for DXA diagnostic criteria and current clinical guidelines for the management of osteoporosis. Errors in BMD measurement, differences in reference populations, and variations in calculation methods used, can all affect the actual T-score and Z-score value. Attempts to standardize these values have made considerable progress, but inconsistencies remain within and across BMD technologies. This can be a source of confusion for clinicians interpreting BMD results. A clear understanding of T-scores and Z-scores is essential for correct interpretation of BMD studies in clinical practice.
Similar content being viewed by others
References
The International Society for Clinical Densitometry. Introduction to understanding bone densitometry. International Society for Clinical Densitometry. Hartford, CT. Available at: http://www.iscd.org/visitors/pdfs/UnderstandingBMDTesting2006-FINAL.ppt. Accessed 24/8/2009.
The International Society for Clinical Densitometry. Official Positions & Pediatric Official Positions. International Society for Clinical Densitometry. Hartford, CT. 2007. Available from: http://www.iscd.org/Visitors/positions/OfficialPositionsText.cfm. Accessed 24/8/2009.
Bilezikian JP, Delmas PD, Johnell O, Miller PD, Pennisi, Rizzoli R. International Osteoporosis Foundation Slide Kit. International Osteoporosis Foundation, Nyon, Switzerland. 2009. Available at: http://www.iofbonehealth.org/health-professionals/educational-tools-and-slide-kits/iof-osteoporosis-teaching-slide-kit.html? Accessed 24/8/2009.
United States, Public Health Service, Office of the Surgeon General. Bone health and osteoporosis: a report of the surgeon general. Washington, DC: U.S. Department of Health and Human Services, Office of the Surgeon General; 2004.
WHO Study Group. Assessment of fracture risk and its application to screening for postmenopausal osteoporosis. World Health Organization; Geneva, Switzerland. 1994. Report No.: 843.
National Osteoporosis Foundation. Clinician’s guide to prevention and treatment of osteoporosis, National Osteoporosis Foundation, Washington, D.C. 2008. Available at: http://www.nof.org/professionals/Clinicians_Guide.htm. Accessed 31/3/2009.
Carey JJ, Delaney MF, Thacker HL. Osteoporosis. In: Falcone T, Hurd WW, editors. Clinical reproductive medicine and surgery. Philadelphia, PA: Mosby Elsevier; 2007. p. 371–85.
Bates DW, Black DM, Cummings SR. Clinical use of bone densitometry: clinical applications. JAMA. 2002;288:1898–900.
Bonnick SL. Bone density in clinical practice. 2nd ed. Totowa, NJ: Humana Press; 2004. p. 69–91.
Faulkner KG. Clinical use of Bone Densitometry. In: Marcus R, Feldman D, Kelsey J, editors. Osteoporosis. San Diego, CA: Academic Press; 2001. p. 433–59.
Kanis JA. Diagnosis of osteoporosis and assessment of fracture risk. Lancet. 2002;359:1929–36.
Kanis JA, Melton LJ III, Christiansen C, Johnston CC, Khaltaev N. The diagnosis of osteoporosis. J Bone Miner Res. 1994;9:1137–41.
American College of Obstetricians and Gynecologists, Women’s Health Care Physicians. ACOG practice bulletin. clinical management guidelines for obstetrician-gynecologists. Obstet Gynecol. 2004;103:203–16.
Hodgson SF, Watts NB, Bilezikian JP, et al. American association of clinical endocrinologists medical guidelines for clinical practice for the prevention and treatment of postmenopausal osteoporosis: 2001 edition, with selected updates for 2003. Endocr Pract. 2003;9:544–64.
Dawson B, Trapp RG. Basic & clinical biostatistics. 4th ed. Hightstown, NJ: McGraw-Hill; 2004.
Looker AC, Wahner HW, Dunn WL, et al. Updated data on proximal femur bone mineral levels of US adults. Osteoporos Int. 1998;8:468–89.
Carey JJ, Delaney MF, Love TE, Richmond BJ, Cromer BA, Miller PD, et al. DXA-generated Z-scores and T-scores may differ substantially and significantly in young adults. J Clin Densitom. 2007;10:351–8.
Bennett HS, Dienstfrey A, Hudson LT, Oreskovic T, Fuerst T, Shepherd J. Standards and measurements for assessing bone health-workshop report co-sponsored by the international society for clinical densitometry (ISCD) and the national institute of standards and technology (NIST). J Clin Densitom. 2006;9:399–405.
Lenchik L, Kiebzak GM, Blunt BA, International Society for Clinical Densitometry Position Development Panel and Scientific Advisory Committee. What is the role of serial bone mineral density measurements in patient management? J Clin Densitom. 2002;5(Suppl):S29–38.
Genant HK, Grampp S, Gluer CC, et al. Universal standardization for dual x-ray absorptiometry: patient and phantom cross-calibration results. J Bone Miner Res. 1994;9:1503–14.
Faulkner KG, Roberts LA, McClung MR. Discrepancies in normative data between lunar and hologic DXA systems. Osteoporos Int. 1996;6:432–6.
Looker AC, Orwoll ES, Johnston CC Jr, et al. Prevalence of low femoral bone density in older U.S. adults from NHANES III. J Bone Miner Res. 1997;12:1761–8.
Binkley N, Kiebzak GM, Lewiecki EM, et al. Recalculation of the NHANES database SD improves T-score agreement and reduces osteoporosis prevalence. J Bone Miner Res. 2005;20:195–201.
Cauley JA, Lui LY, Ensrud KE, et al. Bone mineral density and the risk of incident nonspinal fractures in black and white women. JAMA. 2005;293:2102–8.
Mikuls TR, Saag KG, Curtis J, et al. Prevalence of osteoporosis and osteopenia among african americans with early rheumatoid arthritis: the impact of ethnic-specific normative data. J Natl Med Assoc. 2005;97:1155–60.
Barrett-Connor E, Siris ES, Wehren LE, et al. Osteoporosis and fracture risk in women of different ethnic groups. J Bone Miner Res. 2005;20:185–94.
Cauley JA, Wu L, Wampler NS, et al. Clinical risk factors for fractures in multi-ethnic women: the women’s health initiative. J Bone Miner Res. 2007;22:1816–26.
Henry MJ, Pasco JA, Pocock NA, Nicholson GC, Kotowicz MA. Reference ranges for bone densitometers adopted Australia-wide: geelong osteoporosis study. Australas Radiol. 2004;48:473–5.
Richmond BJ, Carey JJ, Delaney MF, Licata AA, Miller PD, Cromer BA, Love TE. The impact of body weight on DXA generated Z-scores in young adults. 17th International Bone Densitometry Workshop, Kyoto, Japan. 2006. Report No.: 01-09.
Carey JJ, Delaney MF, Love TE, Cromer BA, Miller PD, Richmond BJ, et al. Dual-energy X-ray absorptiometry diagnostic discordance between Z-scores and T-scores in young adults. J Clin Densitom. 2009;12:11–6.
Raisz LG. Clinical practice. Screening for osteoporosis. New Engl J Med. 2005;353:164–71.
Khosla S, Melton LJ. Clinical practice. Osteopenia. New Engl J Med. 2007;356:2293–300.
Rosen CJ. Clinical practice. Postmenopausal osteoporosis. New Engl J Med. 2005;353:595–603.
Ebeling PR. Clinical practice. Osteoporosis in men. New Engl J Med. 2008;358:1474–82.
Hui SL, Slemenda CW, Johnston CC Jr. Age and bone mass as predictors of fracture in a prospective study. J Clin Invest. 1988;81:1804–9.
Melton LJ III, Atkinson EJ, O’Fallon WM, Wahner HW, Riggs BL. Long-term fracture prediction by bone mineral assessed at different skeletal sites. J Bone Miner Res. 1993;8:1227–33.
Marshall D, Johnell O, Wedel H. Meta-analysis of how well measures of bone mineral density predict occurrence of osteoporotic fractures. BMJ. 1996;312:1254–9.
Cummings SR, Nevitt MC, Browner WS, Stone K, Fox KM, Ensrud KE, et al. Risk factors for hip fractures in white women. New Engl J Med. 1995;332:814–5.
WHO Study Group. Prevention and management of osteoporosis. World Health Organization, Geneva, Switzerland. 2003. Report No.: 921.
Kanis JA, Johnell O, Oden A, Johansson H, McCloskey E. FRAXTM and the assessment of fracture probability in men and women from the UK. Osteoporos Int. 2008;19:385–97.
The World Health Organization FRAX® Tool. Available at: http://www.shef.ac.uk/FRAX/. Accessed 25/8/2009.
Cauley JA. Worldwide Epidemiology of Fractures: Difference Between East and West. Proceedings of ISCD Annual Meeting, Orlando, Fla, USA. 2009.
Kanis JA, Burlet N, Cooper C, Delmas PD, Reginster JY, et al. European guidance for the diagnosis and management of osteoporosis in postmenopausal women. Osteoporos Int. 2008;19:399–428.
Mithal A, Singh G, Vadhavkar S, Mannalithara A, Triadafilopoulos G. War on osteoporosis: are we giving back hard-won victories? Arthritis Rheum. 2008;58(Suppl):940.
Blake GM, Fogelman I. Interpretation of bone densitometry studies. Semin Nucl Med. 1997;27:248–60.
Wainwright SA, Marshall LM, Ensrud KE, et al. Hip fracture in women without osteoporosis. J Clin Endocrinol Metab. 2005;90:2787–93.
Siris ES, Chen YT, Abbott TA, Barrett-Connor E, Miller PD, Wehren E, et al. Bone mineral density thresholds for pharmacological intervention to prevent fractures. Arch Intern Med. 2004;164:1108–12.
Lyles KW, Colon-Emeric CS, Magaziner JS, Adachi JD, Pieper CF, et al. Zoledronic acid and clinical fractures and mortality after hip fracture. New Engl J Med. 2007;357:1799–809.
Fletcher RH, Fletcher SW, Wagner EH. Clinical epidemiology the essentials. Baltimore, MD: Williams & Wilkins; 1996.
Sim J, Wright C. Research in health care. Cheltenham, England: Nelson Thornes Ltd; 2002.
Camacho PM, Agrawal M, Norton I, Diaz I. Using DXA Z-Scores to predict the presence of secondary causes of osteoporosis. Proceeding of 28th Meeting American Society for Bone and Mineral Research, Philadelphia, PA. M092. 2006.
Painter SE, Kleerekoper M, Camacho PM. Secondary osteoporosis: a review of the recent evidence. Endocr Pract. 2006;12:436–45.
Sikon AL, Thacker HL, Carey J, Deal C, Licata AA. Secondary osteoporosis: are we recognizing it? J Womens Health (Larchmt). 2006;15:1174–83.
McClung MR, Geusens P, Miller PD, Zippel H, Bensen WG, Roux C, et al. Effect of Risedronate on the risk of hip fracture in elderly women. New Engl J Med. 2001;344:333–40.
Neer RM, Arnaud CD, Zanchetta JR, Prince R, Gaich GA, et al. Effect of parathyroid hormone (1–34) on fractures and bone mineral density in postmenopausal women with osteoporosis. New Engl J Med. 2001;344:1434–41.
Black DM, Delmas PD, Eastell R, Reid IR, Boonen S, Cauley JA, et al. Once yearly Zoledronic acid for treatment of postmenopausal osteoporosis. New Engl J Med. 2007;356:1809–22.
Orwoll E, Ettinger M, Weiss S, Miller P, Kendler D, et al. Alendronate for the treatment of osteoporosis in men. New Engl J Med. 2000;343:604–10.
Dawson-Hughes B, Harris SS, Krall KA, Dallal GE. Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older. N Engl J Med. 1997;337:670–6.
Saag KG, Shane E, Boonen S, Marin F, Donley DW, Taylor KA, et al. Teriparatide or Alendronate in glucocorticoid-induced osteoporosis. New Engl J Med. 2007;357:2028–39.
Levine RJ. Placebo controls in clinical trials of new therapies for osteoporosis. J Bone Miner Res. 2003;18:1154–9.
Brody BA, Dickey N, Ellenberg SS, Heaney RP, Levine RJ, O’Brien RL, et al. Is the use of placebo controls ethically permissible in clinical trials of agents intended to reduce fractures is osteoporosis. J Bone Miner Res. 2003;18:1105–9.
Silverman SL, Cummings SR, Watts NB. Recommendations for the clinical evaluation of agents for treatment of osteoporosis: consensus of an expert panel representing the American Society for Bone and Mineral Research (ASBMR), the International Society for Clinical Densitometry (ISCD), and the National Osteoporosis Foundation (NOF). J Bone Miner Res. 2008;23:159–65.
American College of Rhematology ad hoc committee on glucocorticoid-induced osteoporosis. Recommendations for the prevention and treatment of glucocorticoid-induced osteoporosis. Arthritis Rheum. 2001;44:1496–503.
National Osteoporosis Guideline Group. Guideline for the diagnosis and management of osteoporosis in postmenopausal women and men from the age of 50 years in the UK. 2009. Available from: http://www.shef.ac.uk/NOGG/NOGG_Pocket_Guide_for_Healthcare_Professionals.pdf Accessed: 26/8/2009.
Qaseem A, Snow V, Shekelle P, Hopkins R, Forclea MA, Owens DK. Screening for Osteoporosis in men: a clinical practice guideline from the American College of Physicians. Ann Intern Med. 2008;148:680–4.
Kanis JA, Seeman E, Johnell O, Rizzoli R, Delmas P. The perspective of the International Osteoporosis Foundation on the official positions of the International Society for Clinical Densitometry. J Clin Densitom. 2005;8:145–7.
Lewiecki EM, Miller PD, Leib ES, Bilezikian JP. Response to “the perspective of the International Osteoporosis Foundation on the official positions of the International Society for Clinical Densitometry”, by John A. Kanis et al. J Clin Densitom. 2005;8:143–4.
Watts NB, Lewiecki EM, Miller PD, Baim S, 2008 National Osteoporosis Foundation. Clinician’s guide to prevention and treatment of osteoporosis and the world health organization fracture risk assessment tool (FRAX): what they mean to the bone densitometrist and bone technologist. J Clin Densitom. 2008;11:473–7.
Watts NB. What is osteopenia, and what should be done about it? Cleve Clin J Med. 2006;73:29–32.
Thacker HL, Richmond B. In rebuttal: osteopenia is a useful diagnosis. Cleve Clin J Med. 2006;73:34–8.
Carey JJ. Fracture risk models in clinical practice. 2009. Available at: http://www.iscd.org/visitors/resources/fractureriskmodels.cfm; Accessed: 26/8/2009.
Nelson HD, Helfand M, Woolf SH, Allan JD. Screening for postmenopausal osteoporosis: a review of the evidence for the U.S. Preventive Services Task Force. Ann Intern Med. 2002;137:529–41.
Conflicts of Interest
Drs. John Carey and Miriam Delaney report no conflict of interest for this paper.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Carey, J.J., Delaney, M.F. T-Scores and Z-Scores. Clinic Rev Bone Miner Metab 8, 113–121 (2010). https://doi.org/10.1007/s12018-009-9064-4
Published:
Issue Date:
DOI: https://doi.org/10.1007/s12018-009-9064-4