title: Factors affecting short-term bone density precision assessment and the effect on patient monitoring.
abstract: The most widely used procedure for performing a BMD reproducibility assessment (same-technologist with simple repositioning on the same day) systematically underestimates precision error and will lead to over categorization of change in a large fraction of monitored patients.
  
  
  BACKGROUND
  The most common procedure for establishing the least significant change (LSC) to monitor bone mineral density (BMD) with DXA is for the same technologist to perform repeat subject scans on the same day with simple repositioning. The objective of the current report is to determine how the reproducibility scanning procedure impacts on the precision assessment and categorization of change in routine clinical practice.
  
  
  METHODS
  The study population was drawn from the database of the Manitoba Bone Density Program which includes all clinical DXA test results for the Province of Manitoba, Canada. All patients who had baseline and follow up total spine (L1-4) and the total hip BMD measurements on the same instrument up to March 31, 2007 were included as the 'clinical monitoring population' (N = 5048 scan-pairs). BMD precision was assessed in a convenience sample of patients who were agreeable to undergoing a repeat assessment (50% performed on the same day with repositioning, 68% by different technologists) (N = 331 spine and 328 hip scan-pairs).
  
  
  RESULTS
  Precision error was greater when the scan-pairs were acquired on different days than on the same day for both the total spine (p < .001) and total hip (p < .01). No other factor was consistently associated with precision error. The reference LSC (different days and different technologists) categorized the smallest fraction of the monitored population with change, whereas other combinations gave a significant rate of over categorization (up to 19.3% for the lumbar spine and up to 18.3% for the total hip).
  
  
  CONCLUSIONS
  The most widely procedure for performing a BMD reproducibility assessment (same-technologist with simple repositioning on the same day) systematically underestimates precision error and will lead to over categorization of change in a large fraction of monitored patients.
authors.affiliation: Department of Medicine, University of Manitoba, Winnipeg, Manitoba, Canada. bleslie@sbgh.mb.ca
authors.forename: William D
authors.initials: WD
authors.lastname: Leslie
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods* / Q000592:standards; D015519:Bone Density / Q000502:physiology*; D016208:Databases, Factual / Q000706:statistics & numerical data; D005260:Female; D006615:Hip / Q000000981:diagnostic imaging; D006801:Humans; D008297:Male; D008350:Manitoba; D008875:Middle Aged; D008991:Monitoring, Physiologic; D015203:Reproducibility of Results; D012107:Research Design / Q000592:standards; D013131:Spine / Q000000981:diagnostic imaging; D013997:Time Factors