How is a bone mineral density (BMD) score calculated?

How Bone Mineral Density Scores Are Calculated

Bone mineral density (BMD) scores are calculated by comparing an individual's measured bone density to reference populations, with the results expressed as standard deviations (T-scores and Z-scores) from these reference means. 1, 2

Measurement Process

• BMD is measured using dual-energy X-ray absorptiometry (DXA), which is the gold standard for non-invasive bone density assessment
• DXA measures bone mineral content (BMC) in grams and the area of bone in cm²
• BMD is calculated as BMC divided by bone area, expressed in g/cm² 1
• Common measurement sites include:
  • Lumbar spine (typically L1-L4)
  • Total hip
  • Femoral neck
  • One-third (33%) radius 1, 2

Score Calculation

T-score Calculation

• T-score = (Patient's BMD - Young adult mean BMD) / Standard deviation of young adult population 2
• Represents the number of standard deviations above or below the mean BMD for a young adult reference population 1, 2
• Uses the young adult Caucasian female normative reference database for both women and men 1, 2
• Classification based on T-scores:
  • Normal: T-score ≥ -1.0
  • Osteopenia/low bone mass: T-score between -2.5 and -1.0
  • Osteoporosis: T-score ≤ -2.5 2

Z-score Calculation

• Z-score = (Patient's BMD - Age-matched mean BMD) / Standard deviation of age-matched population 2
• Represents the number of standard deviations above or below the mean BMD for age and sex-matched controls 1, 2
• Recommended for use in premenopausal women and men under age 50 1
• Z-score ≤ -2.0 is defined as "bone mineral density below the expected range for age" 2

Reference Databases and Considerations

• The World Health Organization (WHO) classification system is based on T-scores derived from reference populations 2
• Different DXA manufacturers may use different reference databases, which can affect score calculations 3
• For hip measurements, the National Health and Nutrition Examination Survey III (NHANES III) database is recommended as the reference standard 3
• For spine measurements, manufacturers typically use their own reference databases 3
• In men, female reference databases are recommended for T-score calculation, while male reference databases are used for Z-score calculation 3

Clinical Implications

• Each standard deviation decrease in BMD represents approximately a twofold increase in fracture risk 1
• T-scores and Z-scores can differ significantly in young adults, with differences of up to 1 SD or more in 11% of cases 4
• The choice of regression model used to analyze reference data can affect the calculated peak BMD and standard deviation values 5
• BMD measured by DXA can overestimate BMD in taller individuals and underestimate BMD in petite individuals 2

Important Considerations for Serial Measurements

• Serial measurements should be conducted using the same DXA system, software, scan mode, and patient positioning 1
• Quantitative BMD comparison with serial measurements is based on absolute BMD values in g/cm², not T-scores or Z-scores 1
• The least significant change (LSC) is calculated as 2.77 × precision error, with a 95% confidence level 1
• Only changes that meet or exceed the LSC should be considered significant 1

Understanding how BMD scores are calculated is essential for proper interpretation of results and appropriate clinical decision-making regarding osteoporosis diagnosis and fracture risk assessment.