Defined daily doses of antimicrobials reflect antimicrobial prescriptions in ambulatory care

Background: Data on actual antimicrobial prescriptions are rarely made available and the World Health Organization (WHO) recommends using the number of defined daily doses (DDDs) as a measurement unit for inter-country comparisons of antimicrobial use in outpatients. However, because antimicrobials are given for limited periods of time and the DDDs may not represent the doses used in practice, this measurement unit has been blamed for poorly reflecting the number of antimicrobial prescriptions in outpatients. Methods: For 11 out of 15 European countries, data on national outpatient sales of antimicrobials and on antimicrobial prescriptions to outpatients in 1997 were purchased from the Institute for Medical Statistics (IMS) Health. For two additional countries, i.e. Sweden and Denmark, similar data were obtained from the National Corporation of Swedish Pharmacies and the Danish Medicines Agency, respectively. The relationships between the number of DDDs and the number of prescriptions, on the one hand, and the antimicrobial use density (DDD per 1000 inhabitant-days) and the prescription rate (prescriptions per 1000 inhabitants), on the other hand, were assessed with the two-tailed Spearman coefficient for non-parametric correlations. Results: The number of DDDs, as defined by the WHO, and the number of prescriptions of antimicrobials to outpatients in European countries in 1997 were strongly correlated. Similarly, the antimicrobial use density and the prescription rate in these countries were strongly correlated. These relationships were found for total systemic antimicrobials and for all major antimicrobial classes. Conclusion: Our results confirm the relevance of the number of DDDs per 1000 inhabitant-days as a measurement unit to compare outpatient antimicrobial use among countries or regions.