Guidelines for ATC classification and DDD assignment - WHOCC

Use of the ATC/DDD system allows standardisation of drug groupings and a stable
drug utilization metric to enable comparisons of drug use between countries,
regions, and other health care settings, and to examine trends in drug use over time
and in different settings.
Drug consumption figures should preferably be presented as numbers of
DDDs/1000 inhabitants/day or, when in-hospital drug use is considered, as DDDs
per 100 bed days. Sales or prescription data presented in DDD/1000
inhabitants/day may provide a rough estimate of the proportion of the population
within a defined area treated daily with certain drugs. For example, the figure 10
DDDs/1000 inhabitants/day indicates that 1% of the population on average gets a
certain treatment daily.
For antiinfectives (or other drugs normally used in short periods) it is often
considered most appropriate to present the figures as numbers of DDDs per
inhabitant per year, which will give an estimate of the number of days for which
each inhabitant is, on average, treated annually.
For example, 5 DDDs/inhabitant/year indicates that the consumption is equivalent
to the treatment of every inhabitant with a 5 days 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.
For some drug groups where DDDs have not been established, alternative ways of
presenting data are recommended. For example, consumption of dermatological
preparations can be presented in grams of ointment, cream etc, and antineoplastic
agents ATC group L01 can be presented in grams of active ingredient.
When there is a known discrepancy between the prescribed daily dose (PDD) and
the DDD, it is important to take this into account when interpreting drug
consumption figures. Caution should also be taken in situations where the
recommended dosage differs from one indication to another (e.g. antipsychotics), in
severe versus mild disease (e.g. antibiotics) and where PDDs may differ from one
population to another (e.g., according to sex, age, ethnicity or geographic location).
Finally, it should be taken into considerations that some prescribed medications are
not dispensed, and the patient does not always take all the medications, which are
dispensed. Specially designed studies are required to measure actual drug intake at
the patient level.
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