Guidelines for ATC classification and DDD assignment - WHOCC

Further reviews of DDDs
After the first three years period, the DDD normally remains unchanged for at least
five years unless the WHO Working Group decides to make a total revision of all
DDDs assigned in an ATC group. Proposed DDD changes from users of the
system, based on new information will always be considered, but only after the
three years revision has been performed.
E. Description of other drug utilization metrics
Cost
Drug use can be expressed in terms of costs (e.g. national currency). Cost figures
are suitable for an overall cost analysis of drug expenditure. National and
international comparisons based on cost parameters are often misleading and of
limited value in the evaluation of drug use. Price differences between alternative
preparations and different national cost levels make the evaluation difficult. Longterm
studies are also difficult due to fluctuations in currency and changes in prices.
When cost data are used, an increase in the use of cheaper drugs may have little
influence on the total level, while a shift to more expensive drugs is more readily
noticed.
Volume
Common physical units (e.g. grams, kilos, litres), numbers of packages or tablets
and numbers of prescriptions are also used for quantifying drug consumption.
These units can be applied only when the use of one drug or of well defined
products is evaluated. Problems arise, however, when the consumption of whole
drug groups is considered.
If consumption is given in terms of grams of active ingredients, drugs with low
potency will have a larger fraction of the total than drugs with high potency.
Combined products may also contain different amounts of active ingredients from
plain products, which will not be reflected in the figures.
Counting numbers of tablets also has disadvantages, because strengths of tablets
vary, with the result that low strength preparations contribute relatively more than
high strength preparations. Also, short-acting preparations will often contribute
more than long-acting preparations.
Numbers of prescriptions do not give a good expression of total use, unless total
amounts of drugs per prescription are also considered. Counting of prescriptions,
however, is of great value in measuring the frequency of prescriptions and in
evaluating the clinical use of drugs (e.g. diagnosis and dosages used).
30