Different versions of the anatomical therapeutic chemical classification system and the defined daily dose--are drug utilisation data comparable?

To investigate whether published drug utilisation studies contain satisfactory references to the ATC/DDD version applied and thus are usable for comparative purposes. A literature survey of drug utilisation studies was made. Seventy-three articles were identified by two Medline searches, one in 1996 and another in 1998. The articles were classified into four different groups. Groups 1 and 2 give proper references to the defined daily doses (DDDs) used in the studies, either by full reference to the version of the ATC Index with DDDs or by listing the actual DDDs used. The articles in groups 1 and 2 represent 46% of the articles in the survey. The articles classified in groups 3 and 4 give references only to general articles about the anatomical therapeutic chemical classification (ATC)/DDD system or no references at all. Fifty-four percent of the articles in the survey were classified in these groups. In these articles it is not possible to identify which DDDs have been used in the presentation of drug consumption data. According to the results from our literature survey, it is not common practice to include the ATC codes and the DDD values used or to make reference to the versions of the ATC/DDD index used when results from drug utilisation studies are published. One possible reason for this might be little or no knowledge about the ATC/DDD system as a dynamic system in which alterations are made annually. The lack of references to the actual DDDs used in the studies make comparisons between different data sets difficult and misleading.