Guidelines for ATC classification and DDD assignment ... - WHOCC

Further reviews of DDDsAfter the first three years period, the DDD normally remains unchanged for at leastfive years unless the WHO Working Group decides to make a total revision of allDDDs assigned in an ATC group. Proposed DDD changes from users of thesystem, based on new information will always be considered, but only after thethree years revision has been performed.E. Description of other drug utilization metricsCostDrug use can be expressed in terms of costs (e.g. national currency). Cost figuresare suitable for an overall cost analysis of drug expenditure. National andinternational comparisons based on cost parameters are often misleading and oflimited value in the evaluation of drug use. Price differences between alternativepreparations and different national cost levels make the evaluation difficult. Longtermstudies 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 littleinfluence on the total level, while a shift to more expensive drugs is more readilynoticed.VolumeCommon physical units (e.g. grams, kilos, litres), numbers of packages or tabletsand numbers of prescriptions are also used for quantifying drug consumption.These units can be applied only when the use of one drug or well defined productsis evaluated. Problems arise, however, when the consumption of whole druggroups is considered.If consumption is given in terms of grams of active ingredients, drugs with lowpotency will have a larger fraction of the total than drugs with high potency.Combined products may also contain different amounts of active ingredients fromplain products, which will not be reflected in the figures.Counting numbers of tablets also has disadvantages, because strengths of tabletsvary, with the result that low strength preparations contribute relatively more thanhigh strength preparations. Also, short-acting preparations will often contributemore than long-acting preparations.Numbers of prescriptions do not give a good expression of total use, unless totalamounts of drugs per prescription are also considered. Counting of prescriptions,however, is of great value in measuring the frequency of prescriptions and inevaluating the clinical use of drugs (e.g. diagnosis and dosages used).30