How do you interpret a Dual-Energy X-ray Absorptiometry (DEXA) bone density scan result?

How to Read and Interpret DEXA Bone Density Results

DEXA (Dual-Energy X-ray Absorptiometry) scan results should be interpreted using standardized T-scores and Z-scores, with T-scores ≤ -2.5 indicating osteoporosis, scores between -1.0 and -2.5 indicating osteopenia, and scores ≥ -1.0 considered normal bone density. 1

Understanding T-scores and Z-scores

• T-score compares your bone mineral density (BMD) to that of a healthy young adult of the same sex at peak bone mass, expressed as standard deviations (SD) from this reference 1
• Z-score compares your BMD to people of the same age and sex, expressed as standard deviations 1, 2
• For adults over 50 years, T-scores are the primary diagnostic measurement 1
• For children, adolescents, premenopausal women, and men under 50 years, Z-scores are preferred, with Z-score ≤ -2.0 considered "below the expected range for age" 1

WHO Classification Based on T-scores

• Normal bone density: T-score ≥ -1.0 1
• Osteopenia (low bone mass): T-score between -1.0 and -2.5 1, 3
• Osteoporosis: T-score ≤ -2.5 1
• Severe or established osteoporosis: T-score ≤ -2.5 plus one or more fragility fractures 1

Key Measurement Sites

• Lumbar spine (L1-L4): Primarily trabecular bone, shows early changes in bone density 1
• Hip (total hip and femoral neck): Femoral neck is the reference site for epidemiological studies 1
• Forearm (33% radius): Used when hip or spine cannot be measured 1
• The lowest T-score from any of these sites determines the diagnostic classification 1

Clinical Interpretation Guidelines

• Diagnosis should be based on the lowest T-score from the lumbar spine, femoral neck, total hip, or 33% radius 1
• An osteoporotic fracture supersedes any DXA measurement - patients with fragility fractures should be diagnosed with osteoporosis even if T-scores are in the osteopenic range 1
• Treatment is recommended for all postmenopausal women and men >50 years with a T-score ≤ -2.5 1
• For patients with osteopenia, fracture risk assessment tools like FRAX should be used 1, 2
• Treatment is recommended for patients with a 10-year probability of hip fracture ≥3% or major osteoporotic fracture ≥20% based on FRAX 1

Monitoring Changes Over Time

• Changes should be evaluated using absolute BMD values (g/cm²), not T-scores or Z-scores 1
• The least significant change (LSC) should be calculated for each center (typically 2.8-5.6% depending on precision) 1
• Only changes that meet or exceed the LSC should be considered significant 1
• Patients should ideally return to the same DXA machine for follow-up scans 1

Common Pitfalls to Avoid

• Degenerative changes in the spine can falsely elevate BMD measurements 1
• Artifacts such as surgical hardware, calcifications, or jewelry can affect readings 4
• Extreme body sizes (very tall, very small, or obese patients) may require alternative assessment methods 1
• Different DXA machines may give slightly different results, so follow-up scans should ideally be performed on the same device 1
• Vertebral fracture assessment (VFA) should be considered as DXA alone may miss existing vertebral fractures 1

Alternative Assessment Methods

• Quantitative Computed Tomography (QCT): Provides volumetric BMD but uses different reference values than DXA 1, 5
• Calcaneal ultrasonography: Less sensitive (75% sensitivity, 66% specificity at T-score threshold of -1.0) but radiation-free 1
• FRAX tool: Incorporates clinical risk factors with BMD to calculate 10-year fracture probability 1

By understanding these key elements of DEXA interpretation, clinicians can accurately assess bone health status and make appropriate treatment decisions to reduce fracture risk.