Pitfalls in the measurement and interpretation of bone mineral density

Puneet Mashru MD, S.C.E Rheumatology UK
Head of the Department of Rheumatology, Sir HN Reliance foundation Hospital & Consultant Rheumatologist, Jaslok Hospital and Breach Candy Hospital, Mumbai

Pitfalls of DXA and how to overcome them

Dual-energy X-ray absorptiometry (DXA) is a widely used imaging technology for measuring bone mineral density (BMD) and body composition. However, there are several potential pitfalls and limitations that should be considered when interpreting DXA results.

Technical Factors: DXA scans are highly dependent on the quality of the equipment and the skill of the operator. Any technical errors or inconsistencies in the imaging process can lead to inaccurate results. It is essential to review the scan in detail, including the images, to ensure the quality of the scan. The spine must include part of T12 vertebra up to part of L5 vertebra with both iliac crest visible, and the femur should include the acetabulum, greater trochanter, and extend below the lesser trochanter. Proper positioning should be checked, and any errors or artifacts should be corrected.
Fracture Risk Assessment: BMD is an important component to assess fracture risk, but it accounts for only 50-80% of fracture risk. To better assess a patient’s risk of fracture, other risk factors, such as age, sex, medical history, and history of falls, should be taken into consideration in addition to DXA results. Various fracture assessment tools, such as FRAX, can be used to incorporate these additional factors, but even these tools have their limitations.
Bone Quality Assessment: DXA cannot assess bone quality, such as bone microarchitecture. For example, in type 2 diabetic patients and patients on glucocorticoids, fractures occur at higher BMD values due to poor bone microarchitecture. Additional software, like Trabecular Bone Score (TBS), can be used to assess microarchitecture, which can be entered into FRAX scoring.
DXA may be inaccurate in patients with degenerative disease, aortic calcification, post-fracture, or other conditions that can affect bone density measurements. These areas should be appropriately excluded from the scan. In patients with diffuse idiopathic skeletal hyperostosis (DISH) and ankylosing spondylitis with syndesmophytes, the spine BMD may be inaccurate, and in such cases, the forearm and femur reading can be taken into account, or quantitative computed tomography (QCT) of the spine may be used.
DXA is a two-dimensional measurement (g/cm2) and may overestimate fracture risk in individuals with small body frames and underestimate fracture risk in obese individuals due to the superimposed soft tissue.

Special situations

DXA cannot differentiate non-osteoporotic causes of low BMD like osteomalacia, multiple myeloma, renal bone diseases, and other conditions that affect bone density. Therefore, it is important to look at the patient’s history and additional diagnostic tests to determine the underlying cause of low BMD.

How can you compare DXA scans?

Comparing DXA scans requires calculating the Least Significant Change (LSC) value of the DXA machine. This value represents the smallest statistically significant change in bone mineral density (BMD) and is used to determine whether there has been a significant change in value between two scans. However, some centres may not have this value calculated, which can make comparisons between scans challenging.