Antimicrobial prescribing in hospitals: be careful what you measure

Measurement of prescribing is an important component of antimicrobial stewardship. The standard unit of measurement in hospitals is defined daily doses denominated by bed days (e.g. DDDs per 1000 bed days) but alternatives have not been evaluated in depth. Electronically prescribed doses of systemic antibacterials administered in this trust during 2008 were analysed in order to generate 10 indices of antimicrobial use for each of 14 departments. These indices were five measurements of consumption (DDDs, agent days, courses, antibiotic days and treatment periods) each denominated by two measurements of activity [bed days and finished consultant episodes (FCEs)]. The 10 indices cluster into four groups within which they correlate well but between which correlation is poor. These four groups comprise a volume-related measurement of consumption (DDDs, agent days, antibiotic days) and an exposure-related measurement of consumption (courses, treatment periods), each denominated by either bed occupancy (bed days) or patient throughput (FCEs). Indices within these four groups seem to provide different and complementary information. Restricting measurement of antimicrobial use to a single metric such as DDDs per 1000 bed days may be insufficient. It is not known which (if any) of these indices are the best predictors of antimicrobial-related risks such as resistance pressure or Clostridium difficile infection.