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To the Editor

The International Committee for Standards in Bone Measurement gave final approval to the standardization of proximal femur BMD measurements by DXA in a meeting held in Chicago on December 4, 1996. This follows earlier announcements on adoption of standardized units and spine standardization.1,2 Standardization of femur BMD measurements will include region of interest definition, units of measurement, and reference data.

The region of interest used for femur evaluations will be the Total Femur region of interest as used in NHANES III,3 SOF,4 PEPI,5 and other cross-sectional and prospective studies. These studies have demonstrated that the Total Femur region of interest is equally diagnostic but more precise than the Femoral Neck region of interest predominantly used prior to standardization. As part of the standardization effort, the Total Femur region of interest will be made available by all DXA manufacturers offering Standardized Femur BMD.

The Committee has decided to introduce the term “sBMD,” expressed in mg/cm2, to distinguish Standardized Femur BMD from manufacturer-specific “BMD,” expressed in g/cm2. The equations used to convert manufacturer-specific units [g/cm2] for Total Femur BMD into standardized units [mg/cm2] are as follows:

  • equation image

sBMD values obtained by scanning a patient on any one of these manufacturers' instruments should fall within 3–6% of each other (3% standard error of estimate). These equations were based on the same study used to define sBMD for the spine1 using an approach developed by Lu and colleagues.6 The method used does not require a gold standard and minimizes variance observed between standardized and manufacturer-specific BMD values on all instruments. For new devices entering the market to comply with Standardized Femur BMD a study similar to the one employed here will be required. Such a study would measure a wide range of BMD in human subjects on different scanners and derive Standardized Femur BMD using the method of Lu et al.

The standardization of Total Femur BMD includes the standardization of reference data, thereby making T- and Z-scores derived from different manufacturers' equipment compatible. The data used as the basis for the reference ranges were collected in phases 1 and 2 of the third National Health and Nutrition Examination Survey (NHANES III, 1988–1994).3 The reference data curve was generated based on running means of the raw data. The mean sBMD and standard deviation for the young adult reference for 409 U.S. white women aged 20–29 years was 956 mg/cm2 and 123 mg/cm2, respectively. Age-specific reference data based on a total of 3,251 U.S. white women are listed in Table 1.

Standardized Femur BMD will be made available on newly-shipped devices no later than September 1, 1997, by all participating manufacturers. Users will be provided with an option that will permit Femur BMD to be reported with either the traditional, manufacturer-specific units in g/cm2 (for any of the traditional femur regions of interest) or the standardized, non–manufacturer-specific units in mg/cm2 for the Total Femur region of interest. New systems will be configured to report standardized units by default. Upon request, software will be available from the manufacturers for upgrading existing instruments to provide the standardized units option.

As part of the standardization effort the Committee will collaborate with the relevant scientific and professional societies as well as the regulatory agencies to promote the utilization of Standardized Femur BMD instead of the current, widely-employed, manufacturer-specific Femoral Neck BMD. The medical community should welcome manufacturers' independent T- and Z-scores for diagnosis at the Total Femur region of interest. This will eliminate the currently observed inter-manufacturer discrepancy associated with clinical decision-making based on T- and Z-scores at the femur neck region of interest.

The Committee for Standards in Bone Measurement, a voluntary committee of representatives of both industry and the academic community, has provided users of DXA technology the three critical components for standardization of DXA scan results: standardized units in mg/cm2, a standardized definition of scan regions of interest for spine and femur, and agreement on a universal reference database for the femur, based on NHANES III. This significant accomplishment makes comparisons of data between DXA devices and between manufacturers possible. The clinical benefits of these efforts are evident in greater confidence in scan results, clearer definition of patient condition when applying WHO criteria, ability to compare data between scans performed on different devices, and expanded reference data for the United States. The committee also agreed to update reference data from time-to-time as new information becomes available for additional populations. Standardization of male Total Femur reference data will be completed when the final NHANES data are published. The committee understands that population-based reference data equivalent to NHANES are being evaluated by the European Foundation for Osteoporosis. The need for geographic dependent reference data will be addressed by the committee. The committee will continue its work to expand standardization to other anatomical sites assessed by DXA and will report periodically on its progress using letters to the editor as means for communication.

Table Table 1. STANDARDIZED TOTAL FEMUR REFERENCE DATA FOR WHITE WOMEN
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REFERENCES

  1. Top of page
  2. REFERENCES
  3. COMMITTEE MEMBERS
  • 1
    Genant HK, Grampp S, Glüer CC, Faulkner KG, Jergas M, Engelke K, Hagiwara S, Van Kuijk C 1994 Universal standardization for dual x-ray absorptiometry: Patient and phantom cross-calibration results J Bone Miner Res 9:15031514.
  • 2
    Steiger P 1995 Letter to the editor: Standardization of spine BMD measurements J Bone Miner Res 10:16021603.
  • 3
    Looker AC, Johnston CC Jr., Wahner HW, Dunn WL, Calvo MS, Harris TB, Heyse SP, Lindsay RL 1995 Prevalence of low femoral bone density in older US women from NHANES III J Bone Miner Res 10:796802.
  • 4
    Cummings SR, Black DM, Nevitt MC, Browner W, Cauley J, Ensrud K, Genant HK, Palermo L, Scott J, Vogt TM 1993 Bone density at various sites for prediction of hip fractures Lancet 341:7275.
  • 5
    Bush TL, Wells HB, James MK, Barrett-Connor E, Marcus R, Greendale G, Hunsberger S, McGowan J 1996 Effects of hormone therapy on bone mineral density JAMA 276:13891396.
  • 6
    Lu Y, Ye K, Mathur AK, Hui S, Fuerst T, Genant HK, 1997 Comparative calibration without a gold standard. Stat Med (in press).

COMMITTEE MEMBERS

  1. Top of page
  2. REFERENCES
  3. COMMITTEE MEMBERS

Dieter Felsenberg, M.D.

Freie Universitat Berlin, Radiology, Steglitz Med Centre, Berlin, Germany, for COMAC/BME

Thomas Fuerst, Ph.D.

University of California, San Francisco, CA, U.S.A., Department of Radiology

Harry K. Genant, M.D.

University of California, San Francisco, CA, U.S.A. for the American Society for Bone and Mineral Research and the National Osteoporosis Foundation

Thomas Hangartner, Ph.D.

Wright State University, Dayton, OH, U.S.A. for the American Association of Physicists in Medicine and the National Osteoporosis Foundation

James Hanson, Ph.D.

Lunar Corporation, Madison, WI, U.S.A. - Committee Chairman

Lewis Harrold, VP Engineering

Norland Corporation, Fort Atkinson, WI, U.S.A.

C. Conrad Johnston, Jr., M.D.

Indiana University School of Medicine, Indianapolis, IN, U.S.A., for the National Osteoporosis Foundation

Willi A. Kalender, Ph.D.

University of Erlangen, Germany, for COMAC/BME

Ying Lu, Ph.D.

University of California, San Francisco, CA, U.S.A., Dept. of Radiology

Richard Mazess, Ph.D.

Lunar Corporation, Madison, WI, U.S.A.

Paul D. Miller, M.D.

Colorado Center for Bone Research, P.C., Lakewood, CO, U.S.A., for the International Society of Clinical Densitometry

Rikushi Morita, M.D., Ph.D.

Shiga University, Otsu City, Japan, for the Japanese Society of Bone and Mineral Research

Akira Nakamura, Gen. Mgr.

Aloka Co., Ltd. Tokyo Works, Tokyo, Japan

Russell Nord, Ph.D.

Lunar Corporation, Madison, WI, U.S.A.

Jonathan Reeve, DM, DS, FRCP

Institute of Public Health, University Forvie Site, Cambridge, U.K., for the European Foundation for Osteoporosis

Hans Schiessl

Stratec Medizintechnik GMBH, Pforzheim, Germany

Peter Steiger, Ph.D.

Hologic Inc., Waltham, MA, U.S.A.

Jay Stein, Ph.D.

Hologic Inc., Waltham, MA, U.S.A.

Eric von Stetten, Ph.D.

Hologic Inc., Waltham, MA, U.S.A.

Toshiaki Tamegai Ph.D.

Aloka Co., Ltd., Tokyo, Japan

Georg Tysarczyk-Niemeyer

Stratec Medizintechnik GmbH, Pforzheim, Germany