Sunday, August 30, 2009

Defined Daily Doses and Prescribed Daily Doses

The Defined Daily Doses (DDD) is a measure of prescribing volume maintained by the World Health Organisation (WHO) based upon international prescribing behaviour. It represents the assumed average maintenance dose per day for a drug used for its main indication in adults. The DDD is not a recommended dose but an analytical unit to compare prescribing activity.

The Prescribed Daily Doses (PDD) is the average daily amount of a drug that is actually prescribed by a specified group of prescribers for a given time period.

Defined Daily Doses (DDDs)

For many purposes it is clear that measuring the volume of prescribed drugs in terms of number of items is unsatisfactory. The main problem is that a single item can be of any quantity or for any duration, e.g. six months or one week. There is a need for a system which more reliably measures drug volume.

The system of Defined Daily Doses (DDDs), developed and maintained by the World Health Organisation (WHO) attempts to overcome these problems. In this system each drug is given a value, within its recognised dosage range, that represents "the assumed average maintenance dose per day for a drug used on its main indication in adults."It is emphasised that the DDD is a unit of measurement; it is not a recommended dose and may not be a real dose.

Using this system the amount of an individual drug can be expressed in DDDs and, since the DDD of one drug is assumed to be functionally equivalent to the DDD of any other drug used for a similar purpose, the number of DDDs for two or more such drugs can be added together. It is also possible to add together the DDDs of all the drugs in the same broad therapeutic class or of all the drugs given to one or more patients. By extension, cost per DDD across groups of drugs (a measure of economy) may be compared between practices, health authorities, and regions over time indicating where higher cost alternatives have been used.

Example: A General Practitioner (GP) prescribes 30 10mg tablets of simvastatin. The DDD for simvastatin is 15mg.

Therefore the number of DDDs is (30x10)/15 = 20.

See below for more details.


Section 10.1.1 of the British National Formulary (BNF) is non-steroidal anti-inflammatory drugs (NSAIDs). The following table shows the WHO DDDs for NSAIDs and shows how the total use of these drugs within a practice can be calculated.

BNF NameDDDTotal Quantity Prescribed*Number of DDDs Prescribed
(Total Quantity/DDD)
Diclofenac100mg50,000mg50,000/100 = 500
Ibuprofen1200mg216,000mg216,000/1200 = 180
Naproxen500mg4,800mg4,800/500 = 9.6
Indomethacin100mg2,000mg2,000/100 = 20
Mefenamic acid1000mg10,000mg10,000/1000 = 10
Etodolac400mg4,000mg4,000/400 = 10
The total number of DDDs for NSAIDs prescribed = 816

There are particular groups of drugs where the concept of a DDD is unsuitable:

i. Skin preparations such as ointments or creams whose daily use is heavily influenced by the patients.
ii. Vaccinations and other kinds of 'one-off' treatments.
iii. Drugs that are combined preparations, mixtures and compounds, as DDDs have hitherto been allocated only to single chemicals.
iv. The contraceptive pill and Hormone Replacement Therapy regimens.

DDDs are assigned and reviewed by an international working group convened by the WHO. The group consists of two full-time WHO staff and 11 representatives from nine countries.

To find the DDD value of a drug please visit the World Health Organisationwebsite.

Prescribed Daily Doses (PDDs)

A prescribed daily dose (PDD) for a drug can be determined from prescriptions and medical or pharmacy records. The PDD is the average daily amount of the drug that is actually prescribed by a specified group of prescribers for a given time period. For drugs where the recommended dose differs from one indication to another, it is important that diagnosis is linked to the PDD. Pharmacoepidemiological information (e.g. sex, age) is also important in order to interpret a PDD. The PDD is difficult to determine using Prescription Pricing Division of the Business Services Authority (PPDBSA) data alone as the duration of each item is not available. Therefore, in order to obtain a PDD for English prescribers it is necessary to use data from sample databases, e.g. Intercontinental Medical Statistics (IMS) or General Practice Research Database (GPRD), that contain this information.