DDD Indicators

Introduction to DDD Indicators

Drug Utilization figures expressed in DDDs are generally reported in units that control for population size differences (such as 1000 persons). This provides a measure of exposure or therapeutic intensity in a defined population, allowing comparisons across various time periods and population groups.

Drug Utilization figures should ideally be presented using a relevant denominator for the health context such as numbers of DDDs per 1000 inhabitants per day, DDD per inhabitant per year, or as DDDs per 100 bed days.

DDD per 1000 inhabitants per day: Sales or prescription data presented in DDDs per 1000 inhabitants per day may provide a rough estimate of the proportion of the study population treated daily with a particular drug or group of drugs. The figure 10 DDDs per 1000 inhabitants per day can be interpreted as follows: in a representative group of 1000 inhabitants, 10 DDDs of the drug are utilized on average, on any given day of the year analysed. Alternatively this can be expressed as 10/1000 (1%) of the population are receiving this drug each day in that year. This estimate is most useful for drugs used chronically and when there is good agreement between the average prescribed daily dose (PDD) and the DDD.

DDD per 100 bed days: The DDDs per 100 bed days may be applied when drug use by inpatients is considered. The definition of a bed day may differ between hospitals or countries. A common definition is: A bed day is a day during which a person is confined to a bed and in which the patient stays overnight in a hospital. Day cases (patients admitted for a medical procedure or surgery in the morning and released before the evening) are sometimes included as one bed day and sometimes excluded. The same definition of bed days should always be chosen when performing comparative studies. The figure 70 DDDs per 100 bed days of hypnotics provides an estimate of the therapeutic intensity and estimates that 70% of the inpatients receive one DDD of a hypnotic every day. This measure is applied in analyses of in-hospital drug use. This indicator is quite useful for benchmarking in and between hospitals.

DDD/patient: This indicator is often calculated in pharmacoepidemiological databases and expresses the treatment intensity/total exposure according to a defined study period. If the actual dose used is equivalent to the DDD, the DDD/patient would also express the number of treatment days in a specific period.

Drug utilization data presented in DDDs give a rough estimate of consumption and not an exact picture of the actual drug use, and the estimates described above are only true if there is good agreement between the actually prescribed dose and the DDD.

Calculating DDD indicators

This section is a brief introduction on how to do calculations using the Defined Daily Dose (DDD).

Utilization in DDD

DDD/1000 Inhabitants/day

DDD/100 bed days

DDD/patient

Utilization as a percentage

Utilization of a specific drug as a total percentage of its therapeutic class can be useful to understand its representation. This can be calculated by:

When, for example, looking at the use of a specific antibiotic as a percentage of total antibiotic use, this can be a useful indicator to look at the development of drug resistance in antibiotics.

Expenditure per DDD

This indicator represents the actual cost paid by a health system for specific medicines. This indicator provides information on the actual cost paid for a medicine and allows comparison between countries (international differences in the expenditure for the same medicine). It also allows comparison between medicines licensed with comparable clinical properties and allows the calculation of exact differences within a country or between countries. This indicator must be interpreted with caution if the indications for use differ and if the ratio between the DDD and the prescribed daily dosage differ significantly within a drug group.

Expenditure indicators can be used to find various aspects such as:

  • Pharmaceutical expenditure on total health expenditure
  • Pharmaceutical expenditure per capita
  • Expenditure on medicines on the essential medicines list versus expenditure on non essential medicines
  • Expenditure in specific monetary units (e.g. US dollars, euros) per DDD
  • Expenditure for generics on total pharmaceutical expenditure
  • Expenditure for new medicines on total pharmaceutical expenditure
  • Top ten therapeutic or pharmacological classes by ATC levels
  • Top ten ingredients by 5th level of ATC

Quality indicators based on the ATC/DDD system

With access to drug utilization statistics based on the ATC/DDD system, it is possible to construct simple indicators reflecting the quality of prescribing or drug use. Analogous to the examples above, the adherence to prescribing guidelines can be assessed using the percentage of use of the recommended drug of the total use of drugs in a certain group

The recommended drugs, as well as the drug group, can be defined using ATC codes. Several disease and patient-specific factors may influence drug choice, and more specifically not choosing the recommended drug in the individual case. Thus, the indicator does not include patient specific factors such as indication, severity of disease, lack of effect of first-line treatment, adverse effects or contraindications, and this should be considered in the interpretation. Despite these limitations, simple indicators have been useful to demonstrate potential quality problems and large variations in prescribing and drug utilization.

The Drug Utilization 90% (DU90%) indicator has been proposed as a means of focusing on the most commonly used drugs (1). This indicator measures the number of drugs accounting for 90% of the use in DDDs. Products are ranked in order of DDDs and the number of drugs accounting for 90% of use is the DU90%. A good way to represent this is by plotting the products in rank order of DDDs on the x-axis against the number of defined daily doses of each product on the y-axis. The focus is on the drugs that account for 90% of the volume and adherence to treatment guidelines within this 90%. The adherence to guidelines can then be calculated as the number of DDDs which are in the treatment guidelines over all DDDs within the 90%. Caution is needed when the ratio between the DDD and the prescribed daily dose differ largely between the drugs included in the indicator.

1. Bergman U1, Popa C, Tomson Y, Wettermark B, Einarson TR, Aberg H, Sjöqvist F. Drug utilization 90%--a simple method for assessing the quality of drug prescribing. Eur J Clin Pharmacol. 1998 Apr;54(2):113-8