What do the results of a DEXA (Dual-Energy X-ray Absorptiometry) scan indicate?

What DEXA Bone Density Results Indicate

DEXA scan results measure bone mineral density (BMD) and report it as T-scores and Z-scores, which diagnose osteoporosis, assess fracture risk, and guide treatment decisions. 1

Understanding the Scoring Systems

T-Score Interpretation (Primary Metric for Adults >50)

• Normal bone density: T-score ≥ -1.0 1
• Osteopenia (low bone mass): T-score between -1.0 and -2.5 2, 1
• Osteoporosis: T-score ≤ -2.5 2, 1
• Severe/established osteoporosis: T-score ≤ -2.5 plus one or more fragility fractures 1

The T-score compares your BMD to a healthy young adult of the same sex at peak bone mass, expressed as standard deviations from this reference population. 1 Each standard deviation decrease in bone density approximately doubles fracture risk in untreated postmenopausal women. 3

Z-Score Interpretation (For Specific Populations)

• Use Z-scores for: children, adolescents, premenopausal women, and men under 50 years 1
• Below expected range for age: Z-score ≤ -2.0 1
• Z-scores compare BMD to people of the same age and sex 1

Clinical Decision-Making Based on Results

Diagnosis Protocol

Base your diagnosis on the lowest T-score from any of these sites: lumbar spine, femoral neck, total hip, or 33% radius. 1 An osteoporotic fracture supersedes any DXA measurement and confirms the diagnosis regardless of BMD values. 1

Treatment Thresholds

• Treat all postmenopausal women and men >50 years with T-score ≤ -2.5 1
• For osteopenia patients: Use FRAX (WHO Fracture Risk Algorithm) to calculate 10-year fracture probability 2, 1
• Treat if FRAX shows: ≥3% 10-year probability of hip fracture OR ≥20% probability of major osteoporotic fracture 1

Monitoring Changes Over Time

Evaluate changes using absolute BMD values in g/cm², NOT T-scores or Z-scores. 1 This is critical because T-scores can be misleading when tracking treatment response or disease progression.

• Calculate the "least significant change" specific to your DXA center 1
• Only changes meeting or exceeding this threshold are clinically significant 1
• Patients should return to the same DXA machine for follow-up scans to ensure accuracy 1

Critical Pitfalls and Limitations

Degenerative Spine Changes

DEXA systematically overestimates BMD in patients with lumbar arthritis, osteophytes, facet joint hypertrophy, or aortic calcification. 4 This affects approximately 40% of women aged 55 and 85% of those >75 years. 4

Warning signs of artifact interference:

• T-score differences >1.0 between adjacent vertebrae 4
• Unexpectedly high lumbar spine BMD compared to hip measurements 4

Solutions when severe lumbar arthritis is present:

• First choice: Quantitative CT (QCT) of lumbar spine and hip (appropriateness rating 8/9) 4
• Alternative: Measure distal forearm instead (appropriateness rating 7/9) 4

Other Technical Limitations

• Results vary between different DXA machines and manufacturers 2, 5
• Extreme body sizes may require alternative methods like QCT or calcaneal ultrasonography 1
• Requires very low radiation exposure (1 µSv), equivalent to one-tenth of a chest x-ray 2

Alternative Imaging When DEXA is Inadequate

For patients with advanced degenerative spine changes or scoliosis, QCT provides more accurate volumetric BMD assessment. 4 Opportunistic CT scans (performed for other indications) can screen for osteoporosis using Hounsfield units: HU values >160 indicate low osteoporosis risk, while HU <110 correlate significantly with osteoporosis. 2