Available online 5 December 2022

Dual-energy X-ray absorptiometry (DXA) is used for osteoporosis diagnosis, fracture prediction and to monitor changes in bone mineral density (BMD). Change in DXA instrumentation requires formal cross-calibration and procedures have been described by the International Society for Clinical Densitometry. Whether procedures used for BMD cross-calibration are sufficient to ensure lumbar spine trabecular bone score (TBS) cross-calibration is currently uncertain. The Manitoba Bone Density Program underwent a program-wide upgrade in DXA instrumentation from GE Prodigy to iDXA in 2012, and a representative a sample of 108 clinic patients were scanned on both instruments. Lumbar spine TBS (L1-L4) measurements were retrospectively derived in 2013. TBS calibration phantoms were not available at our site when this was performed. We found excellent agreement for lumbar spine BMD, without deviation from the line of perfect agreement, and low random error (standard error of the estimate [SEE] 2.54% of the mean). In contrast, spine TBS (L1-L4) showed significant deviation from the line of identity: TBS(iDXA) = 0.730 x TBS(Prodigy) + 0.372 (p<0.001 for slope and intercept); SEE 5.12% of the mean with negative bias (r=-0.550). Results were worse for scans acquired in thick versus standard mode, but similar when the population was stratified as BMI < or > 35 kg/m2. In summary, it cannot be assumed that just because BMD cross-calibration is good that this applies to TBS. This supports the need for using TBS phantom calibration to accommodate between-scanner differences as part of the manufacturer's TBS software installation.

Since the introduction of dual-energy X-ray absorptiometry (DXA) for osteoporosis diagnosis, fracture prediction and to monitor changes in bone mineral density (BMD) over time in untreated patients and those receiving anti-osteoporosis treatment1. Detecting small changes in BMD in requires stable DXA performance and careful attention to scanning technique to minimize measurement error. Ideally, repeat measurements should be performed on the same DXA scanner as the baseline measurement due to between-instrument differences in calibration, even for instruments from the same manufacturer and model. Replacing DXA instrumentation requires careful planning to assess the quantitative impact of scanner change on BMD measurements, and procedures for performing this have been described by the International Society for Clinical Densitometry (ISCD)2,3.

Lumbar spine trabecular bone score (TBS), a grey-level texture measure derived from spine dual-energy x-ray absorptiometry (DXA) images, is a BMD-independent risk factor for fracture4. The use of TBS for guiding patient management is supported by guidelines from several organizations5, 6, 7. TBS was cleared by the Food and Drug Administration (FDA) in 2012. Calibration is part of the TBS manufacturer's installation process and the regulatory approval for clinical use of TBS using a specially designed fractal phantom. Whether DXA cross-calibration procedures used for BMD are sufficient to ensure TBS cross-calibration is currently uncertain.

The Manitoba Bone Density Program underwent a program-wide upgrade in DXA instrumentation from GE Prodigy to iDXA in 2012. As part of that procedure, a representative a sample of patients were scanned on both instruments to derive BMD cross-calibration measurements. We retrospectively evaluated the level of TBS agreement between GE Prodigy and iDXA scanners.

In Canada, health services including DXA testing are provided to nearly all residents through a single public health care system8. DXA testing through the Manitoba Density Program has been managed as an integrated program since 19979. As part of a program-wide upgrade in DXA instrumentation and cross-calibration procedure, we recruited a convenience sample from among patients referred for DXA assessment, who were agreeable to a second DXA measurement and who did not have severe structural

The study population characteristics are summarized in Table 1. In general, these 108 patients reflect those typically referred for BMD testing in our program, median age 63.7 years, 97% female, 98% White. A small fraction of scans (n=20, 19%) were acquired on thick mode and none were acquired on thin mode. A smaller fraction of the population (n=12, 10%) had BMI exceeding 35 kg/m². The median absolute difference in spine BMD L1-L4 between Prodigy and iDXA scanners was 0.017 g/cm2

We found excellent agreement between GE Prodigy and iDXA scanners for lumbar spine BMD, without a detectable shift in overall calibration or deviation from line of perfect agreement. In contrast, spine TBS showed significant differences in all measured parameters, even when restricted to standard acquisition mode and/ or BMI in the range ≤35 kg/m². A large portion of this difference is likely related to the absence of a TBS calibration phantom at the time this study was performed. This

This study had no external funding body.

William Leslie declares no conflict of interest.

Didier Hans: Co-ownership in the TBS patent. Stock options or royalties: Med-Imaps. Research grants: Amgen, Agnovos, GE Healthcare.

Authors' roles: conception, design, data analysis, drafting the article (WDL), interpretation of data (All Authors); critically revising the article for important intellectual content (All Authors); final approval of the version to be published (All Authors); and agreement to be accountable for all aspects of the work (All Authors). WDL had full access to all the data in the study and takes the responsibility for the integrity of the data and the accuracy of the data analysis.

This article has been reviewed and approved by the members of the Manitoba Bone Density Program Committee.

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© 2022 The International Society for Clinical Densitometry. Published by Elsevier Inc.