Why is a DXA (dual-energy X-ray absorptiometry) bone density scan performed on only one side of the body?

Why DXA Scans Are Performed on One Side of the Body

DXA scans of the hip and forearm are performed on only one side because bone mineral density (BMD) is generally symmetrical between left and right sides, making unilateral measurement sufficient for diagnosis while minimizing radiation exposure and scan time. 1

The Standard Practice: Non-Dominant Side Measurement

The current guideline-based approach specifies scanning the non-dominant side for both hip and forearm measurements 1:

• Hip scanning: The non-dominant hip should be measured routinely, consistent with the NHANES study methodology which used the left side 1
• Forearm scanning: The non-dominant arm is the standard site when forearm DXA is indicated 1
• Consistency requirement: The same side must be used for all longitudinal follow-up measurements to ensure accurate monitoring of BMD changes over time 1

Why Bilateral Symmetry Makes Unilateral Scanning Sufficient

In general, there is no significant difference in BMD between both femurs in most patients, which is the fundamental reason why scanning only one side is medically appropriate 1. This bilateral symmetry allows for:

• Reduced radiation exposure to the patient 1
• Shorter scan acquisition time 1
• Lower cost without compromising diagnostic accuracy 2

Important Exceptions: When the Contralateral Side Should Be Scanned

Bilateral asymmetry can occur in specific clinical situations, and awareness of these exceptions is critical 1:

• Stroke patients: May have differential loading of lower extremities 1
• Prolonged immobility: Can cause unilateral bone loss 1
• Hip arthroplasty or hardware: If one hip has a prosthesis or osteosynthesis material, the contralateral side must be used 1
• Prior fracture or surgery: The unaffected side should be measured 1

The Lumbar Spine: A Different Approach

Unlike the hip and forearm, the lumbar spine is scanned centrally (not unilaterally) because it is a midline structure 1. The posterior-anterior (PA) spine scan includes L1-L4 vertebrae and does not involve side selection 1.

Common Pitfalls to Avoid

Failing to document which side was scanned can compromise longitudinal monitoring 1. Key practice points include:

• Always document whether the right or left side was measured 1
• Use the same side for all follow-up scans to ensure precision in detecting BMD changes 1
• If the initially scanned side becomes unsuitable (due to fracture, surgery, or hardware), clearly document the switch to the contralateral side and recognize that direct comparison may be limited 1
• Be aware that some guidelines recommend measuring both femurs, though the general consensus favors the non-dominant hip approach 1

Clinical Implications for Fracture Risk Assessment

Hip BMD from a single side provides sufficient diagnostic and prognostic information 2. Research demonstrates that:

• Femoral neck BMD shows the highest odds ratio for identifying patients with vertebral fractures 2
• Hip BMD demonstrates better discriminative ability than lumbar spine for detecting osteoporosis in elderly subjects 2
• Adding spine measurements to hip measurements does not significantly increase fracture risk prediction 2