[Defined daily doses in drugs with combinations of active principles: differences between three calculation methods.]

The defined daily dose (DDD) is a measurement unit of drug consumption associated with the Anatomic Therapeutic Chemical (ATC) classification for its use in drug utilization studies. Due to the frequent marketing of pharmaceutical specialties with active ingredients combination, the results of the calculation of DDD product in combination with several active ingredients may vary depending on various possible calculation methods. The aim of this study was to compare different ways to calculate DDD of two groups of drugs that included monodrugs as well as combined products.

From the prescription billing data during 2019 in Catalonia, the results obtained by three methods when calculating the DDDs of non-insulin hypoglycaemic drugs (nIHG) and drugs used in obstructive respiratory pathology (ORP) were compared. The three methods used were the reference calculation provided by the World Health Organization, the calculation of the considered main ingredient and the individualized calculation of all active ingredients of the combination. These methods were compared using the Wilcoxon test for paired data.

The results obtained showed high differences both in the total DDD and in the percentage of participation of each pharmacological subgroup within the studied groups. Differences of 17% were observed in nIHG, and of 118% in ORP drugs. The calculation system that takes into account all the active ingredients of the combinations gives a more approximate idea of the total drug consumption, as well as the relative weight of each subgroup.

The calculation of all the active ingredients included in the specialties with drug combinations seems to be the one that can be most useful in pharmacoepidemiological management scenarios such as those in our environment.