OsteoEd

Practice Cases

Bone Density Measurement Basics: Question Five

Your answer is correct.

Her confirmatory DXA shows total hip bone mineral density 0.93 SD above the 66-year-old mean and 0.36 SD below the 25-year-old mean. Which of the statements concerning her report is correct?

  • Option A Her average/total hip T-score is +0.93
  • Option B Her average/total hip T-score is -0.36
  • Option C Her average/total hip Z-score is -0.36
  • Option D She has osteoporosis

< back to case

What is a T-score? What is a Z-score?

  • A T-SCORE is the number of standard deviations the bone mineral density measurement is above or below the YOUNG-NORMAL MEAN bone mineral density.
  • A Z-SCORE is the number of standard deviations the measurement is above or below the AGE-MATCHED MEAN bone mineral density.

The T- and Z-scores were developed because of variation in BMD measurement technology among different manufacturers. Therefore, the BMD results are expressed as standard deviations from a comparison to the referent mean.

T-Score Image Copyright Osteoporosis International (Used with permission) (1)

T-scores are commonly used to define osteoporosis/osteopenia. A BMD more than 2.5 standard deviations below the mean for a young healthy adult white woman identifies 30 percent of all postmenopausal women as having osteoporosis; half of these women will already have had a fracture. The hip T-score is the site used in clinical decisions.

Z-score is less commonly used but may be helpful in identifying persons who should undergo a work-up for secondary causes of osteoporosis. A Z-score changes over time in relation to the T-score.

The following shows how one might interconvert T- and Z-scores.

Converting T-score to Z-score at the hip:

  • Age 50: T = Z - 0.37
  • Age 60: T = Z - 1.01
  • Age 70: T = Z - 1.56
  • Age 80: T = Z - 2.11
  • Age 90: T = Z - 2.52
  1. steoporosis. Cost-effectiveness analysis and review of the evidence for prevention, diagnosis and treatment. Osteoporos Int 8 1998; 10: S001-S002.

< back to case


Warning: date(): It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected the timezone 'UTC' for now, but please set date.timezone to select your timezone. in /nfs/bronfs/uwfs/hw00/d65/osteoed/inc/functions.php on line 179