BMD Reference Information and Definitions

  • T-score: number of standard deviations above (+) or below (-) mean peak young adult bone density. Used for individuals >50y age.
  • Z-score: number of standard deviations above (+) or below (-) the mean density for an individual of same age and sex. Used for individuals <50y age CAROC 2013 revision.
  • LSC: Least Significant Change: amount by which one BMD value must differ from another for the difference to be statistically significant to 95% level of confidence. This corresponds to 2.77 times the precision error for single measurements at each time point for 95% certainty.
  • Mayfair BMD technologist-specific precision error is 0.010 g/cm2 with LSC considered to be 0.028 g/cm2 for 95% certainty. When there is less bone density, there is less precision.

*** IMPORTANT *** NEW RECOMMENDED DIAGNOSTIC CATEGORIESCAROC 2013 REVISED ***:

For women and men =>50y : NORMAL (T-score greater than or equal to -1.0), LOW BONE MASS (previously OSTEOPENIA) (T-score -1.1 to -2.4), and OSTEOPOROSIS (T-score less than or equal to -2.5). 

For women and men < 50y and children <18y, the diagnosis of osteoporosis should not be made on the basis of BMD alone. In these age groups, Z-score above or below -2.0 is used to categorize density WITHIN EXPECTED RANGE FOR AGE (Z-score greater than -2.0) or density BELOW EXPECTED RANGE FOR AGE (Z-score less than or equal to -2.0). 

*** Also under the current revised guidelines, Z score now reported for individuals under age 50 with PREVIOUS guidelines using T-score for femur and spine areas in this age group (unless pediatric). A change in Z score compare to prior T score when comparing to past examination may not indicate a significant change in density. Instead refer to specific category and density change comments.

*** IMPORTANT *** NEW FRACTURE RISK GUIDELINES CAROC 2013REVISED ***:

  • 10 year fracture risk (CAROC 2010): In 2010/2013, the Canadian Association of Radiologists and the Osteoporosis Society of Canada (CAROC) updated guidelines including a “Ten Year Fracture Risk” determination. This applies only to those =>50y (both male and female).
  • Categories are: LOW (<10%), MODERATE (10% to 20%), or HIGH (>20%). This predicts fracture risk of the hip, spine, forearm or proximal humerus, based ONLY upon the T-score of the FEMURNECK, taking AGE and SEX into account derived from a white female reference database. PREVIOUS guidelines required the determination of 10 year fracture risk based upon the lowest T-score found in all of the femur and spine areas, and therefore a change in risk category when comparing to past examination may not indicate a significant change in density. Instead refer to specific density change comments. No risk assessment available for individuals 85yo.
  • Individuals with fragility fracture after age 40 or systemic glucocorticoid therapy >7.5 mg/day > 90 days total in past year have at least moderate fracture risk. If both of these factors occur together, the risk is high.
  • Individuals with fragility hip or vertebral fracture after age 40 or 2 or more fragility fractures after age 40 have high fracture risk regardless of BMD result.
  • Individuals with lumbar spine T-score lower than -2.5 significantly worse than femur neck T-score have moderate fracture risk.
  • Fracture risk may be lower than calculated if osteoporosis drug therapy is effective.

Reference:

Osteoporosis Canada - Tools and Resources
2010 Clinical Practice Guidelines for Diagnosis and Management of Osteoporosis in Canada:Summary 
2013 CAR Practice Guidelines Technical Standard BMD Reporting Mayfair Mar 2014

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