Standardization of Central DXA Absolute BMD Values
In November 1990, the major manufacturers of DXA equipment agreed to work together in the area of standards as part of an international committee, known as the International Committee for Standards in Bone Measurement. Under the auspices of this committee, a study of 100 healthy women (10) was performed in which each of the women underwent PA spine and proximal femur studies on the Hologic QDR-2000, the Norland XR-26 Mark II, and the Lunar DPX-L. The women ranged in age from 20 to 80, with an average age of 52.6 years. The difference in PA lumbar spine BMD was greatest between the Norland XR-26 and the Lunar DPX-L, averaging 0.118 g/cm2 or 12.2%. The average difference between the Lunar DPX-L and the Hologic QDR-2000 was 0.113 g/cm2 or 11.7%. The average difference in BMD at the PA lumbar spine between the Norland XR-26 and the Hologic QDR-2000 was the smallest at only 0.012 g/cm2 or 1.3%.
Based on this data, equations were derived for the conversion of PA lumbar spine BMD obtained on any one of these manufacturer's machines to the BMD that would be expected to be obtained on each of the other two. The equations for each of the three pairs of scanners are shown in Table 5-2.
Table 5-3
Formulas for the Conversion of Manufacturer-Specific PA Spine BMDs to the Standardized BMD (sBMD)
sBMDSPINE = 1000 (1.0761 x Norland XR-26 BMDSPINE) sBMDSPINE = 1000 (0.9522 x Lunar DPX-L BMDSPINE) sBMDSPINE = 1000 (1.0755 x Hologic QDR-2000 BMDSPINE)
Adapted from ref. 9 with permission from the American Society for Bone and Mineral Research.
Table 5-4
Conversion Formulas for BMDs in the Femoral Neck Between DXA Devices
Hologic QDR-2000 NeckBMD = (0.836 x Lunar DPX-L NeckBMD) - 0.008
Hologic QDR-2000 NeckBMD = (0.836 x Norland XR 26 NeckBMD) + 0.051
Lunar DPX-L NeckBMD = (1.013 x Hologic QDR 2000 NeckBMD) + 0.142
Lunar DPX-L NeckBMD = (0.945 x Norland XR 26 NeckBMD) + 0.115
Norland XR-26 NeckBMD = (0.961 x Lunar DPX-L NeckBMD) - 0.037 Norland XR-26 NeckBMD = (1.030 x Hologic QDR 2000 NeckBMD) + 0.058
Adapted from ref. 10 with permission from the American Society for Bone and Mineral Research.
In order derive an equation to convert each manufacturer's reported BMD to a standardized BMD (sBMD), the ESP2 was scanned on each of the three devices. Equations were developed based on the BMD value for the central vertebra of the ESP averaging 1000 mg/cm2 when scanned on each of the devices. These formulas are shown in Table 5-3. In November 1994, the Committee for Standards in DXA approved these formulas for the standardization of PA spine BMD values (11). Using these formulas, a PA spine sBMD obtained by scanning a patient on a Norland, Hologic, or Lunar DXA device should be within 2 to 5% of the sBMD that would be obtained when the patient was scanned on either of the other two devices.
The value for the sBMD is multiplied by 1000 to convert it to mg/cm2 rather than reporting it as g/cm2. This serves to readily distinguish the sBMD from the nonstan-dardized value. In other words, if the BMD obtained in the spine on a Lunar DPX-L is 0.985 g/cm2, this value becomes 938 mg/cm2 when reported as the sBMD (0.985 x 0.9522 = 0.9379 g/cm2 x 1000 = 938 mg/cm2). Using these formulas to convert the average spine BMD in the study population to the sBMD, the differences in BMD between the three machines were greatly reduced. Instead of an average difference of 12.2% between the Norland and Lunar values, the difference in the sBMD was only 2.8%. The difference between Hologic and Lunar was reduced to 2.2% and the difference between Hologic and Norland was 2.7%.
Conversion formulas were also developed by Genant et al. (10) for the femoral neck for each pair of scanners. These formulas are shown in Table 5-4.
In December 1996, the International Committee for Standards in Bone Measurement2 approved formulas for the sBMD for the total femur region of interest (12). The total
1 See Chapter 4 for a discussion of the ESP.
2 Formerly the Committee for Standards in DXA.
Table 5-5
Formulas for the Conversion of Manufacturer-Specific Total Femur BMD to the Standardized BMD (sBMD)
sBMDtotal femur = 1000 [(1.008 x Hologic BMDtotal femur) + 0.006] sBMDtotal femur = 1000 [(0.979 x Lunar BMDtotal femur) - 0.031] sBMDtotal femur = 1000 [(1.012 x Norland BMDtotal femur) + 0.026]
femur region of interest includes the femoral neck, Ward's area, the trochanter, and the shaft of the proximal femur. This region appears to have equal diagnostic utility but better precision than the femoral neck. The formulas for the sBMD for the total femur, shown in Table 5-5 were based on the work by Genant et al. (10) from which the formulas for sBMD of the spine were also derived. The sBMD from any one of the three central DXA devices should fall within 3 to 6% of the sBMD on any of the other two.
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