Dual-energy X-ray absorptiometry diagnostic discordance between Z-scores and T-scores in a young Iranian population

BACKGROUND Dual-energy X-ray Absorptiometry (DXA) is considered the gold standard for non-invasive measurement of bone mass. T-scores and Z-scores are used to present the results of bone mass. The present study was designed to evaluate the discordance between T-scores and Z-scores calculated at a same level and its relation with age, gender and body mass index (BMI) in a representative sample of normal population. METHODS This cross-sectional study was conducted as a part of a comprehensive survey, Iranian Multicenter Osteoporosis Study (IMOS), designed to assess bone health among healthy adults. Each individual underwent both L1-L4 antero-posterior lumbar spine and hip DXA scan. The difference between the T- and Z-scores measured at each of the four skeletal sites was then calculated. RESULTS A -1.21 to 1.21 point difference was noted in the Z- and T- scores measured at each site. While the difference between the T-and Z-scores was less than 0.5 SD in most of the cases, the difference was higher than 1 SD in about 5% of the subjects. CONCLUSION Standardization of Z-score definition and calculation techniques as well as developing an ethnicity-matched reference population is needed to improve the reliability of DXA-generated Z-scores.