(2003; 49 pages)

### 6.1. The concept of the defined daily dose (DDD)

Definition; DDDs per 1000 inhibitants per day; DDDs per 100 bed-days; DDDs per inhibitant per year

The historical development of the concept of the defined daily dose (DDD) and its early applications are described in the Preface.

**The DDD is the assumed average maintenance dose per day for a drug used for its main indication in adults.**

It should be emphasized that the DDD is a unit of measurement and does not necessarily correspond to the recommended or prescribed daily dose (PDD). Doses for individual patients and patient groups will often differ from the DDD as they must be based on individual characteristics (e.g. age and weight) and pharmacokinetic considerations.

The DDD is often a compromise based on a review of the available information about doses used in various countries. The DDD may even be a dose that is seldom prescribed, because it is an average of two or more commonly used dose sizes.

Drug utilization figures should ideally be presented as numbers of DDDs per 1000 inhabitants per day or, when drug use by inpatients is considered, as DDDs per 100 bed-days. For antiinfectives (or other drugs normally used for short periods), it is often considered to be most appropriate to present the figures as numbers of DDDs per inhabitant per year.

These terms are explained below.

*DDDs per 1000 inhabitants per day*

Sales or prescription data presented in DDDs per 1000 inhabitants per day may provide a rough estimate of the proportion of the study population treated daily with a particular drug or group of drugs. As an example, the figure 10 DDDs per 1000 inhabitants per day indicates that 1% of the population on average might receive a certain drug or group of drugs daily. This estimate is most useful for chronically used drugs when there is good agreement between the average prescribed daily dose (see below) and the DDD. It may also be important to consider the size of the population used as the denominator. Usually the general utilization is calculated for the total population including all age groups, but some drug groups have very limited use among people below the age of 45 years. To correct for differences in utilization due to differing age structures between countries, simple age adjustments can be made by using the number of inhabitants in the relevant age group as the denominator.

*DDDs per 100 bed-days*

The DDDs per 100 bed-days may be applied when drug use by inpatients is considered. The definition of a bed-day may differ between hospitals or countries, and bed-days should be adjusted for occupancy rate. The same definition should be used when performing comparative studies. As an example, 70 DDDs per 100 bed-days of hypnotics provide an estimate of the therapeutic intensity and suggests that 70% of the inpatients might receive a DDD of a hypnotic every day. This unit is quite useful for benchmarking in hospitals.

*DDDs per inhabitant per year*

The DDDs per inhabitant per year may give an estimate of the average number of days for which each inhabitant is treated annually. For example, an estimate of five DDDs per inhabitant per year indicates that the utilization is equivalent to the treatment of every inhabitant with a five-day course during a certain year.

Alternatively, if the standard treatment period is known, the total number of DDDs can be calculated as the number of treatment courses, and the number of treatment courses can then be related to the total population.