Dual-Energy X-ray Absorptiometry Diagnostic Discordance Between Z-Scores and T-Scores in Young Adults

Diagnostic criteria for postmenopausal osteoporosis using central dual-energy X-ray absorptiometry (DXA) T-scores have been widely accepted. The validity of these criteria for other populations, including premenopausal Women and young men, has not been established. The International Society for Clinical Densitometry (ISCD) recommends using DXA Z-scores, not T-scores, for diagnosis in premenopausal women and men aged 20-49 yr, though Studies supporting this position have not been published. We examined diagnostic agreement between DXA-generated T-scores and Z-scores in a cohort of men and women aged 20-49 yr, using 1994 World Health Organization and 2005 ISCD DXA criteria. Four thousand two hundred and seventy-five unique subjects were available for analysis. The agreement between DXA T-scores and Z-scores was moderate (Cohens kappa: 0.53-0.75). The use of Z-scores resulted in significantly fewer (McNemars p 0.001) subjects diagnosed with osteopenia, low bone mass for age, or osteoporosis. Thirty-nine percent of Hologic (Hologic, Inc., Bedford, MA) subjects and 30% of Lunar (GE Lunar, GE Madison. WI) subjects diagnosed with osteoporosis by T-score were reclassified as either normal or osteopenia when their Z-score was used. Substitution of DXA Z-scores for T-scores results in significant diagnostic disagreement and significantly fewer persons being diagnosed with low bone mineral density.