4.2.1 Defined daily dose

In human health, the WHO has recommended a unit of measurement for AMU for surveillance purposes called a defined daily dose (DDD). The DDD is the ‘assumed average maintenance dose for a drug (not necessarily an antibiotic) used for its main indication in adults’. DDDs are assigned to medicines that have a Anatomical Therapeutic Chemical (ATC) code, and each drug is assumed to have one main route of administration (e.g. oral). This means that the actual variety of AMU including different routes of administration and different typical doses and schedules, is not captured by the DDD. A DDD is not the same as the recommended dose for any given patient (as this varies between children and adults, for example). Because of how the DDD is calculated, it is not used by clinicians as it is not relevant to clinical care.

There are several indicators that do use DDD to measure AMU. For example, a common DDD indicator for AMU in hospital inpatients is the number of DDDs per 100 occupied bed-days. This is calculated as:

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An equation: number of packages used multiplied by number of DDD in a package over number of occupied bed-days, all multiplied by 100.

The number of DDD in a package is looked up from standard reference tables maintained by the WHO.

More information about DDD indicators and how to calculate them is available online [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .

The main advantage of the DDD is that it allows for consistent comparisons in AMU between different settings and over time. This makes it particularly useful for comparing AMU between countries, evaluating the effectiveness of antimicrobial stewardship programs from different sites, and monitoring changes in use of a particular class of antimicrobial from one year to the next.