title: Improving the assessment of prescribing: use of a 'substitution index'.
abstract: OBJECTIVE
  To analyse the current and potential utilization of generic drugs in Japan, to examine the maximum possible cost savings from generic drug use and to develop a fairer measure to assess the level of generic drug substitution.
  
  
  METHODS
  We conducted a cross-sectional retrospective analysis of nine million dispensing records during January to March 2010 in Kyoto Prefecture. Maximum potential quantity-based shares were defined as the quantity of generic drugs used plus the quantity of branded drugs that could have been replaced by generic drugs divided by the quantity of all drugs dispensed. We developed a 'substitution index', defined as the proportion of generic drugs out of the total drugs substitutable with generic drugs (based on quantity rather than cost).
  
  
  RESULTS
  Generic drugs had a quantity-based share of 17.9%, a cost-based share of 8.9% and a maximum potential quantity-based share of 50.1%, which is lower than the actual generic drug shares of some other countries. The maximum possible cost savings as a result of generic drug substitution was 16.5%. We also observed wide variations in maximum potential quantity-based shares between health care sectors and health care institutions.
  
  
  CONCLUSIONS
  Simple comparisons based on quantity-based shares may misrepresent the actual generic drug use. A substitution index that takes into account the maximum potential quantity-based share of generic drugs as a fairer measure may promote more realistic goals and encourage generic drug usage.
authors.affiliation: Department of Healthcare Economics and Quality Management, Graduate School of Medicine, Kyoto University, Japan.
authors.forename: Susumu
authors.initials: S
authors.lastname: Kunisawa
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Otsubo
authors.forename: Jason
authors.initials: J
authors.lastname: Lee
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Imanaka
mesh_terms: D017046:Cost Savings; D003430:Cross-Sectional Studies; D016208:Databases, Factual; D057915:Drug Substitution / Q000191:economics*; D016568:Drugs, Generic / Q000191:economics / Q000627:therapeutic use*; D006801:Humans; D007564:Japan; D055553:Prescription Drugs / Q000191:economics / Q000627:therapeutic use*; D012189:Retrospective Studies; D013810:Therapeutic Equivalency
chemical_list: D016568:Drugs, Generic; D055553:Prescription Drugs
keywords: generic drugs; prescribing; substitution index