Drug consumption can be expressed in cost, number of units, number of prescriptions or by the physical quantity of drugs. However these variables can vary between regions and countries over time. This limits comparisons of drug consumption at an international level. To address this, a technical unit of measurement, the Defined Daily Dose (DDD) was created.
Defined Daily Dose (DDD): The assumed average maintenance dose per day for a drug used for its main indication in adults.
DDDs are only assigned for medicines given an ATC codes. The DDDs are allocated to drugs by the WHO Collaborating Centre in Oslo, working in close association with the WHO International Working Group on Drug Statistics Methodology.
Only one DDD is assigned per ATC code and route of administration (e.g. oral formulation). The DDD is sometimes a dose that is rarely or never prescribed because it is an average of two or more commonly used doses.
DDDs are not established for all medicines with an ATC code. Major drug groups without DDDs are topical products (most products in ATC group D), sera (ATC group J06), vaccines (ATC group J07), antineoplastic agents (ATC group L01), general and local anesthetics (ATC group N01), ophthalmologicals and otologicals (most products in ATC group S), allergen extracts (ATC group V01) and contrast media (ATC group V08).
The DDD is a unit of measurement and does not necessarily correspond to the recommended or Prescribed Daily Dose (PDD). Therapeutic doses for individual patients and patient groups will often differ from the DDD as they will be based on individual characteristics such as age, weight, ethnic differences, type and severity of disease, and pharmacokinetic considerations.
Drug utilization data presented in DDDs give a rough estimate of consumption and not an exact picture of actual use. DDDs provide a fixed unit of measurement independent of price, currencies, package size and strength enabling the researcher to assess trends in drug utilization and to perform comparisons between population groups.
By applying DDD it is possible to:
- Examine changes in drug utilization over time
- Make international comparisons
- Evaluate the effect of an intervention on drug use
- Document the relative therapy intensity with various groups of drugs
- Follow the changes in the use of a class of drugs
- Evaluate regulatory effects and effects of interventions on prescribing patterns.
DDD for Children
DDDs are normally assigned based on use in adults.
For medical products approved for use in children, the dose recommendations will differ based on age and body weight. Many medical products used in children are not approved by regulatory agencies for such use, and the usual documentation used by the WHO Collaborating Centre regarding dose regimens is not available.
Paediatric DDDs are challenging to assign and problems related to Drug Utilization Research (DUR) in children cannot be solved by such means.
Estimating prevalence of drug use in children is not possible by using crude sales data presented in DDDs owing to the variability of children’s doses. When undertaking DUR in a population of children the prescribed daily dosages and indications in that population should be obtained if available and compared with the DDD values. If these parameters in the paediatric subgroup are difficult to identify, the general DDD can be used as a standardized measuring tool for overall comparisons, with caveats or limitations associated with using an adult based DDD.
Prescribed Daily Dose (PDD)
It is important to underline that the DDD is a technical unit (fixed unit of measurement) and does not necessarily correspond to the recommended or prescribed daily dose (PDD).
The prescribed daily dose (PDD) is defined as the average dose prescribed according to a representative sample of prescriptions. The PDD can be determined from studies of prescriptions, medical or pharmacy records, and it is important to relate the PDD to the diagnosis on which the drug is used. 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 DDD, it is important to take this into consideration when evaluating and interpreting drug utilization figures.
For drugs where the recommended dosage differs for different indications (e.g. 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 policies and practices. For example, the PDDs of anti-infectives may vary according to the severity of the infection. There are also international differences between PDDs, which can be up to four or five fold higher/lower. PDDs in Asian populations are often lower than in Caucasian populations.