Common Questions
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.
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
- steoporosis. Cost-effectiveness analysis and review of the evidence for prevention, diagnosis and treatment. Osteoporos Int 8 1998; 10: S001-S002.