What a Bone Density Scan Checks
A bone density scan (DXA) measures bone mineral density (BMD) to diagnose osteoporosis, predict fracture risk, and monitor treatment response. 1
Primary Measurements
DXA quantifies the amount of mineralized bone tissue per unit area at specific skeletal sites, expressed as grams per square centimeter (g/cm²). 1 The scan produces two critical scores:
- T-score: Compares your BMD to a healthy young adult reference population of the same sex, expressed as standard deviations above or below the mean 1
- Z-score: Compares your BMD to age-matched peers, used to detect secondary causes of bone loss 1
Standard Scanning Sites
The routine DXA examination measures two primary locations: 1
- Lumbar spine (L1-L4): Frontal projection measuring up to 4 vertebral bodies 1
- Hip: Frontal projection measuring the femoral neck and total hip regions 1
- Distal one-third radius: Used as a third site when hips are unavailable or in hyperparathyroidism cases 1
Diagnostic Thresholds
The World Health Organization defines osteoporosis based on T-scores derived from DXA measurements: 1
- Normal bone density: T-score ≥ -1.0 1
- Osteopenia (low bone mass): T-score between -1.0 and -2.5 1
- Osteoporosis: T-score ≤ -2.5 1
Fracture Risk Prediction
DXA measurements accurately predict future fracture risk, with epidemiological studies demonstrating that BMD correlates directly with population fracture risk and the amount of force necessary to fracture bone. 1 The scan results are incorporated into the FRAX algorithm, which calculates 10-year probability of hip fracture and major osteoporotic fractures by combining BMD with clinical risk factors including age, sex, family history, smoking, steroid use, rheumatoid arthritis, and alcohol consumption. 1
Treatment Monitoring
DXA serves as the primary tool for monitoring response to osteoporosis treatment, with the spine being the most sensitive site for detecting changes. 1 The accuracy and reproducibility of DXA measurements allow clinicians to track whether pharmacologic interventions are effectively maintaining or improving bone density. 1
Common Pitfalls
Artifacts can significantly bias BMD estimates, particularly in the lumbar spine: 1
- Degenerative changes (facet joint osteoarthritis, osteophytes, spondylosis) spuriously increase BMD values 1
- Vertebral fractures demonstrate falsely elevated BMD due to trabecular impaction 1
- Aortic calcifications overlying vertebrae artificially raise measurements 1
- Surgical hardware or implantable devices make sites unsuitable for assessment 1
When more than 2 vertebral levels require exclusion due to artifacts, the second hip or distal radius should be substituted for spine measurements. 1