Guidelines for ATC classification and DDD assignment - WHOCC

Prescribed daily dose
The prescribed daily dose (PDD) can be determined from prescription studies,
medical- or pharmacy records and patient interviews. It is important to relate the
PDD to the diagnosis on which the dosage is based. The PDD will give the average
daily amount of a drug that is actually prescribed. When there is a substantial
discrepancy between the PDD and the defined daily dose (DDD), it is important to
take this into consideration when evaluating and interpreting drug consumption
figures.
For drugs where the recommended dosage differs from one indication to another
(e.g. the antipsychotics) it is important that diagnosis is linked to the prescribed
daily dose given. Pharmacoepidemiological information (e.g. sex, age and
mono/combined therapy) is also important in order to interpret a PDD.
The PDD can vary according to both the illness treated and national therapy
traditions. For the antiinfectives, for instance, PDDs vary according to the severity
of the infection. There are also substantial differences between PDDs in various
countries, which can be up to a four to five fold range. PDDs in Asian populations
are often lower than in Caucasian populations.
The fact that PDDs may differ from one country to another should always be
considered when making international comparisons.
It should be noted that the prescribed daily dose does not necessarily reflect actual
dose consumed.
IV. USE AND MISUSE OF THE ATC/DDD SYSTEM
The main purpose of the ATC/DDD system is as a tool for presenting drug
utilization statistics with the aim of improving drug use. This is the purpose for
which the system was developed and it is with this purpose in mind that all
decisions about ATC/DDD classification are made. Consequently, using the
system for other purposes can be inappropriate. The system has been used since the
early 1970s in drug utilization studies where it has been demonstrated to be suitable
for national and international comparisons of drug utilization, for the evaluation of
long term trends in drug use, for assessing the impact of certain events on drug use
and for providing denominator data in investigations of drug safety (see also point
I C The purpose of the ATC/DDD system, page 14).
National implementation of the ATC/DDD methodology
For monitoring and comparing drug use internationally it is important to ensure that
the data retrieved are comparable. In order to facilitate data collection it is
recommended to establish national medicinal product registries. It is recommended
to have a common structure of these registries. The ATC code and DDD should be
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