ANTIBIOTIC CONSUMPTION; ALTERNATIVE UNITS OF MEASUREMENT

2. ANTIBIOTIC CONSUMPTION; ALTERNATIVE UNITS OF MEASUREMENT

2.1. Units used but not recommended

Various units of measurement have been used to express antibiotic con- sumption, with data coming from numerous sources. Most meaningful units are made up of both a numerator and a denominator, where the numerator measures the amount of antibiotic used and the denominator controls for the size of the population studied. It is the choice of numerator however, which is fundamental to accurately express and compare antibiotic use.

Antibiotic use data are commonly presented in terms of financial expendi- ture (Silber et al., 1994). Although costs allow for an overall analysis of drug expenditure, there are many disadvantages. Specific hospitals negotiate local prices and different suppliers offer different prices therefore, prices are not comparable between either hospitals or countries. Even at a local level, prices often change periodically and are therefore not appropriate to monitor use over time. Furthermore, data based on costs supplies little, or no, information on indication, route of administration, dose, dosing regimen, and duration of treatment. Even if numbers of packages sold/used may be independent of sale prices they may vary with the manufacturer of the country of purchase.

Number of packages of antibiotic sold, is also often quoted in literature. The reason for this is that the data are relatively easily accessed via organisations such as Intercontinental Marketing Services (IMS). IMS is pharmaceutical industry-based and carries out syndicated market research studies. Their data are obtained from drug manufacturers, wholesalers, retailers, pharmacies, mail order, long-term care facilities, and hospitals. IMS claims to be the world’s leading source of information and data analysis for anyone within the pharma- ceutical and healthcare industries. The organisation uses the European Pharmaceutical Market Research Association classification of medicinal prod- ucts, which is not compatible with professional classification schemes used by those who work with antibiotics. The IMS data do not take into account that packages of individual antibiotics may vary with respect to number of unit doses per package, dosages, and route of administration. Again, this means that the data are not appropriate for comparing data between hospitals and countries.

Notably, the oft-quoted data presented by Cars et al. (2001) was originally obtained from IMS, although it was converted into a more appropriate unit of

Quantitative Measurement of Antibiotic Use 107 measurement. Cars and co-workers subsequently published further data from

the same set of countries (Mölstad et al., 2002). Again, the majority of the data were obtained from IMS. They highlighted that the IMS national data were extrapolated from samples of data collected in the individual countries and concluded that it was not possible to validate the data as the IMS data collec- tion method was not fully transparent.

At a local level, it is more often than not the case that data are available at the whole hospital level or, at best, at ward level. Currently, only a minority of hospitals possess a database of patient-level antibiotic prescriptions even though this should be the most accurate data possible. The majority of hospi- tals can supply antibiotic data expressed as amounts purchased by the phar- macy for the whole hospital or amount of antibiotics distributed to specific wards although this does not take into account the amounts disposed of due to expiry past the use-by date. The number of patients receiving an antibiotic pre- scription has been quoted in the past. This is most appropriately quoted for patients in the community, as it is relatively safe to assume that the number of people exposed to antibiotics can be calculated from the number of antibiotic prescriptions dispensed. Unlike hospitalised patients, relatively few patients in the community receive more than one prescription and few receive combina- tion therapy. The number of patients receiving an antibiotic is seldom quoted for hospitalised patients. In addition to the reasons stated above, it is not recom- mended as no quantitative data on dosing and duration of treatment are known and once again, the data from these sources are not comparable. Although num- ber of patients exposed to an antibiotic has been quoted in a few studies, it is really of use only in prevalence studies of infection (Gastmeier et al., 2000). From an ecological standpoint, it is conceivable that it may be an interesting measurement, but it is not recommended for publication of comparative data.

2.2. The recommended unit of antibiotic consumption

A unit of measurement and method of data handling independent of sales prices and package sizes is preferable. Reliable data on antibiotic consumption should ideally be based on individual patient prescriptions—but these data are generally not available. An acceptable compromise is the provision of hospital data which can be broken down by ward/discipline/prescriber. Despite the use of various unsuitable units for measuring antibiotic use in the past, we are mov- ing close to a consensus unit of measurement and data collection. One system has gained a legitimacy and objectiveness—over and above the rest. The con- sensus numerator for measuring antibiotic use is the “defined daily dose” or “DDD.” The consensus denominators are 1,000 inhabitant-days and 100/1,000

108 Fiona M. MacKenzie and Ian M. Gould bed-days for community and hospitalised patients, respectively. The World

Health Organisation (WHO) has met the challenge of standardising the presen- tation of antibiotic use data in order to monitor and benchmark use of this important class of drugs and to correlate this with the emerging problem of antibiotic resistance.