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

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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.

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