Common mistakes in the clinical use of bone mineral density testing

Abstract

Bone mineral density (BMD) testing is used to diagnose osteoporosis, assess fracture risk and monitor changes in BMD over time. A variety of devices and technologies are used to measure BMD or other surrogate markers of bone strength. Measurements obtained with these devices are often reported according to different proprietary standards, and the comparability of values obtained with different instruments is often poor. In addition, there is a high degree of variability in the skills of the technologists performing the tests and the clinicians interpreting the results. Heterogeneity in the guidelines for using BMD measurements together with poor-quality BMD testing and reporting can result in inappropriate clinical decisions, causing unnecessary worry and expense for the patient and possible harm due to unnecessary treatment or treatment being withheld. This Review describes and discusses the mistakes commonly made in BMD testing, and emphasizes the importance of maintaining high-quality standards in order to optimize patient management.

Figure 1

DXA of the lumbar spine with poor positioning. This spine is tilted off-center. In order to obtain a valid bone mineral density measurement the lumbar spine should be perpendicular to the long edges of the DXA table. This patient should be repositioned and rescanned. Abbreviation: DXA, dual-energy X-ray absorptiometry.

Figure 2

Differences in vertebral body labeling. Comparison of lumbar spine BMD with differences in vertebral body labeling is like comparing ‘apples with oranges’. Comparison of the L1–L4 BMD of these two studies showed an apparent loss of BMD at the time of the second DXA. When the vertebral bodies were labeled consistently, BMD was found to be stable. (A) The first DXA scan gives a BMD of 0.767 g/cm². (B) The second scan gives a BMD of 0.730 g/cm². Vertebral body labeling is different to that in the first DXA scan, leading to an invalid BMD comparison. Abbreviations: BMD, bone mineral density; DXA, dual-energy X-ray absorptiometry.

Figure 3

Femoral neck box placement. Each manufacturer has its own standards for correct femoral neck box placement. The dual-energy X-ray absorptiometry technologist must be familiar with the recommendations for the instrument that is used and place the neck box in the same position in serial studies. Here, both images are the same, with the femoral neck box correctly placed on the right. If these were different scans done to evaluate possible changes over time, any comparison would be invalid. (A) Incorrect analysis: femoral neck T-score = −3.2. (B) Correct analysis: femoral neck T-score = −3.0.