What is a DEXA Scan Used For?
A DEXA (dual-energy X-ray absorptiometry) scan is primarily used to diagnose osteoporosis by measuring bone mineral density at the hip and spine, assess fracture risk, monitor treatment response, and measure body composition including lean and fat mass. 1, 2
Primary Clinical Applications
Osteoporosis Diagnosis and Screening
- DEXA is the gold-standard method for diagnosing osteoporosis by measuring bone mineral density (BMD) at central skeletal sites including the total hip, femoral neck, and lumbar spine. 1, 3
- The USPSTF recommends screening all women aged 65 years and older, and postmenopausal women younger than 65 with risk factors (low body weight, parental hip fracture history, smoking, excess alcohol use). 1
- Diagnosis is based on T-scores: normal (≥ -1.0), osteopenia (-1.0 to -2.5), and osteoporosis (≤ -2.5). 2
- The lowest T-score from the lumbar spine, femoral neck, total hip, or 33% radius determines the diagnosis. 2
Fracture Risk Assessment
- DEXA provides critical input for fracture risk calculators like FRAX, which predicts 10-year probability of hip fracture or major osteoporotic fracture. 1
- Risk prediction accuracy improves significantly when BMD from DEXA is incorporated into FRAX calculations compared to clinical risk factors alone. 1
- Treatment is recommended when FRAX shows ≥3% 10-year hip fracture risk or ≥20% major osteoporotic fracture risk. 2
Treatment Monitoring
- DEXA tracks BMD changes over time to assess treatment effectiveness, typically with follow-up scans every 2-3 years. 2
- Changes must be evaluated using absolute BMD values (g/cm²), not T-scores or Z-scores, and must exceed the facility-specific Least Significant Change to be considered real. 2
- Patients should return to the same DXA machine with consistent positioning for valid comparisons. 2
Secondary Applications
Body Composition Analysis
- DEXA accurately measures fat mass, lean mass, and visceral adipose tissue with very low radiation exposure. 4
- Used for diagnosing sarcopenia through lean mass indices and evaluating obesity patterns via android/gynoid fat ratios. 4
- Applications include monitoring lipodystrophy in HIV+ patients and assessing body composition changes in metabolic disorders. 5, 4
Additional Assessments
- Vertebral Fracture Assessment (VFA) should be performed in patients ≥50 years with T-score <-1.0, historical height loss ≥4 cm, or prospective height loss ≥2 cm. 2
- Full Femur Imaging screens for atypical femur fractures in patients on long-term bisphosphonates. 2
Technical Advantages
- DEXA uses very low radiation doses while providing accurate and precise measurements. 5, 6
- The technique correlates well with bone strength and clinical fracture outcomes. 1
- Fracture risk at a specific skeletal site is best predicted when bone density is measured at that exact site. 1
Important Clinical Considerations
Population-Specific Interpretation
- For postmenopausal women and men over 50, T-scores are the primary diagnostic measurement. 2
- For children, adolescents, premenopausal women, and men under 50, Z-scores are preferred, with Z-score ≤ -2.0 considered "below expected range for age." 2
Common Pitfalls to Avoid
- Degenerative spine changes, surgical hardware, and vascular calcifications can falsely elevate BMD measurements. 2, 7
- Vertebrae with structural abnormalities must be excluded from analysis. 2
- Extreme body sizes may require alternative methods like quantitative CT or calcaneal ultrasonography. 2
Quality Standards
- High-quality DXA testing requires meticulous attention to instrument calibration, patient positioning, data acquisition, analysis, and interpretation to avoid misdiagnosis and inappropriate treatment decisions. 6
- Verify correct patient positioning with body and limbs aligned to midline and minimal rotation. 2