Prescribing measures - Volume Measures & Denominators

 Volume Measures 

Prescribing measures

Data for the majority of the NPIs is captured from prescription pricing data and therefore reflects items that have been dispensed. The data does not include items that have been prescribed but not presented for dispensing. This data can be used in a variety of ways to demonstrate the volume of prescribing.

Prescription items

Prescription items refers to the number of items on a prescription. The use of items as a prescribing measure is appropriate for acute treatments such as antibiotics and vaccines, as well as inhalers and creams, as it is a measure of how often a prescriber has decided to write a prescription. The number of prescription items does not take into account the quantity prescribed.

Defined daily doses (DDDs)

Measuring the volume of prescribing in terms of items is unsatisfactory in many cases. In order to overcome this, the World Health Organisation developed the ‘Defined Daily Dose’, or DDD. This is a unit of measure that represents the assumed average maintenance dose per day, of a medicine, when used for its main indication in adults. It is not necessarily a recommended or real dose.  As one DDD is assumed to be functionally equivalent to the DDD of another medicine used for a similar purpose, the number of DDDs for two or more such drugs can be added together. This has the advantage of enabling medicines in the same therapeutic class to be compared.

Average daily quantities (ADQs)

Average daily quantities, or ADQs, are an extension of  DDDs and used for medicines where the DDD was found to be unsatisfactory in the UK, due to differences between UK and international prescribing. ADQs take into account the DDD, but also consider the Prescribed Daily Dose which reflects actual usage by GPs. Again, ADQs are a unit of measure, rather than an actual dose.


The volume and cost of prescribing within an organisation is influenced by the size and nature of the population served, therefore to enable comparison between different health boards, clusters and GP practices, there needs to be a way of weighting prescribing data to take this into account.


Data can be presented per 1,000 patients, however this is only useful if monitoring something that is not influenced by age and gender.

Prescribing Units (PUs)

Prescribing Units (PUs), were introduced to reflect the greater need of older patients for medication. Patients aged 65 and over are counted as 3 prescribing units, and patients under 65 are counted as 1.

Specific therapeutic-group age-sex related prescribing units (STAR-PUs)

STAR-PUs were developed for therapeutic areas where there are differences in the age and sex of patients for whom drugs in specific therapeutic groups are usually prescribed, for example antibiotics.

Both PUs and STAR-PUs were developed in England, however work has been undertaken to ensure these measures are relevant to the Welsh population and use of these measures enables benchmarking with Clinical Commissioning Groups in England.

Targets and thresholds

NPI targets are based on the principle of encouraging all health boards, clusters and GP practices to improve prescribing. For the majority of NPIs, this means encouraging movement towards achieving prescribing rates in the best quartile. For each NPI, the threshold of this quartile is determined by the best performing 25% of practices for the quarter ending December 2020. Targets for each NPI are highlighted throughout the module.


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