T-Scores and Z-Scores

Bone mineral density (BMD) can be measured by a variety of techniques at several skeletal sites. Once measured, the manufacturers’ software uses the BMD to calculate a T-score and/or Z-score. Both T-scores and Z-scores are derived by comparison to a reference population on a standard deviation scale. The recommended reference group for the T-score is a young gender-matched population at peak bone mass, while the Z-score should be derived from an age-matched reference population. T-scores and Z-scores are widely quoted in scientific publications on osteoporosis and BMD studies, and are the values used for DXA diagnostic criteria and current clinical guidelines for the management of osteoporosis. Errors in BMD measurement, differences in reference populations, and variations in calculation methods used, can all affect the actual T-score and Z-score value. Attempts to standardize these values have made considerable progress, but inconsistencies remain within and across BMD technologies. This can be a source of confusion for clinicians interpreting BMD results. A clear understanding of T-scores and Z-scores is essential for correct interpretation of BMD studies in clinical practice.