title: Discordance of longitudinal changes in bone density between densitometers.
abstract: This study examined the concordance in BMD measurement and longitudinal change in BMD between the GE Lunar Prodigy and GE Lunar DPX. Even though a high concordance between the densitometers was observed on a single measurement occasion, a significant discordance in longitudinal changes in BMD was observed.
  
  
  BACKGROUND
  Measurement of bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA) technology plays an important role in the diagnosis and management of osteoporosis. The present study examined the concordance in BMD measurement and longitudinal change in BMD between GE Lunar Prodigy and DPX.
  
  
  METHODS
  BMD at the lumbar spine and femoral neck was measured in 135 individuals (47 men and 88 women, mean age 73+/-9 years) using both GE Lunar DPX and Prodigy densitometers at baseline. In this group, 56 individuals (22 men and 34 women) had repeated BMD measurements using the DPX and Prodigy during a subsequent follow-up visit (average duration: 2.2 years).
  
  
  RESULTS
  For a single BMD measurement, the coefficient of concordance between the Prodigy and DPX was greater than 0.98 at the lumbar spine and 0.96 at the femoral neck, with the slope of linear regression being approximately 1.0. During the period of follow-up, the lumbar spine BMD decreased by -0.5% (S.D. 1.8%) when measured by DPX, which was significantly different (p=0.002) from the change measured by Prodigy (mean change=0, S.D. 2.0%). However, there was no significant difference (p=0.95) in the rate of change in femoral neck BMD measured by DPX (mean=-1.6%, S.D.=2.9) and Prodigy (mean=-1%, S.D.=1.8%). The correlation in rates of BMD change between Prodigy and DPX was 0.63 at the lumbar spine and 0.52 at the femoral neck. Simulation analysis showed that the theoretical maximum correlation in rates of BMD change between Prodigy and DPX was 0.71.
  
  
  CONCLUSIONS
  Despite both densitometers being highly concordant in a single BMD measurement, discordance in the assessment of BMD changes between the Prodigy and DPX densitometers was observed. These findings have implications regarding the assessment of response to therapy in a multi-centre setting when different densitometers are used.
authors.affiliation: Bone and Mineral Research Program, Garvan Institute of Medical Research, St Vincent's Hospital, Sydney, NSW, Australia.
authors.forename: S A
authors.initials: SA
authors.lastname: Frost
authors.forename: N D
authors.initials: ND
authors.lastname: Nguyen
authors.forename: J R
authors.initials: JR
authors.lastname: Center
authors.forename: J A
authors.initials: JA
authors.lastname: Eisman
authors.forename: T V
authors.initials: TV
authors.lastname: Nguyen
mesh_terms: D000368:Aged; D015519:Bone Density / Q000502:physiology*; D003198:Computer Simulation; D003720:Densitometry / Q000592:standards*; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D012727:Sex Characteristics