Defined Daily Doses (DDDs)

This step introduces what Defined Daily Doses are and how to calculate them.

In this next section we will be looking the most commonly used tool to quantitatively measure the efficacy of an AMS programme – defined daily doses (DDDs).

In step 3.4, we looked at some of the outcome measures used to evaluate an AMS programme, and one of these steps was DDDs. The DDD is the assumed average maintenance dose per day for a drug used for its main indication in adults.

DDDs for all medicines are assigned by the WHO and have been used as a standardised measure since the 1970s, although they were not designed to monitor the impact of AMS interventions.

DDD is a technical unit of measurement and does not necessarily reflect the recommended dosage within guidelines or the Prescribed Daily Dose (a patient-focused measure). Therefore, DDDs should not be used to guide treatments for an individual patient. Treatments have to be based on both the infection being treated and individual characteristics (e.g., age, weight, and pharmacokinetic considerations).

DDDs are primarily used by antimicrobial teams to monitor trends within a ward, hospital, or primary care setting. They can be used to measure antibiotic use over time, between locations, and before and after interventions or changes in practice.

The diagram below highlights some of the main advantages and disadvantages of DDDs.

How to calculate DDD

It can be tricky to calculate DDDs, so in this section, we will break down exactly how the calculation is done to make it as easy as possible for you to implement this tool in your own setting.

The diagram below shows how you can work out how many DDDs are in a course of medication.

• The numerator is the grams of an agent either purchased, dispensed, or consumed in a time period, i.e. the total amount of antibiotic used based on the ATC code*, which can be found online.
• ATC Code: Anatomical Therapeutic Chemical code: a medical coding and classification system maintained by the World Health Organization (WHO).
• The denominator is the total number of patient days within that period.
• The multiplier is x1000 patient days. DDD per 100 patient days is usually the most used quantitative measure of antibiotic use in adult medicine. The data needed to calculate it are available in many settings (unlike days of therapy); individual-level data is not needed.

This example shows a typical prescription for a lower urinary tract infection in a female:

Trimethoprim 200mg twice daily for three days

Daily dose = 2 x 200mg = 0.4g

The DDD for oral trimethoprim is 0.4g

Total DDDs for this course is 0.4 divided by 0.4 multiplied by 3 = 3

Data from such DDD calculations can provide the contribution of a treatment episode to overall antibiotic use in both hospital and community settings. In practice, data will come from computerised Pharmacy dispensing data or hospital purchasing/stock control systems. However, if such IT systems are not available, then prescription charts or pharmacy ledgers could be used for manual data collection.

Using DDDs with a denominator

Total or ‘raw’ DDDs at the ward or hospital level are useful to tell whether antibiotic use is increasing, decreasing, or staying the same. However, changes in patient numbers or case mix may account for some of any trends observed, so using a denominator will allow a more meaningful comparison over time. Denominators can also allow comparison between different wards in a hospital, between different directorates in a hospital, or even between hospitals within a geographic area or between countries. The most commonly used denominators in hospital ASPs are the number of patient days or occupied bed days, which require information on patient activity or bed utilisation. This is usually obtained from the hospital administration system. DDD/1000 patient days is recommended by IDSA as a metric for hospital-based ASPs. To help with DDD calculations using a denominator, an online tool is available.

Now it’s your turn to have a go!

Example

Consider a course of oral amoxicillin, 500 milligrams, 3 times daily for 5 days.

How many DDDs is this?

Try to work it out before reading on to the solution.

Solution

Daily dose is 3 (doses per day) x 500 (mg). This gives a total of 1.5 (milligrams per day). The DDD for oral amoxicillin is 1.5 grams (as per WHO ATC index).

Total DDD = 1.5 (daily dose) divided by 1.5 (DDD for amoxicillin), multiplied by 5 (course length in days). = 5 DDD.

Back in step 3.4, we looked at some other quantitative outcome measures to evaluate your ASP. In our next step, we will look at one of these alternatives to DDD – Days of Therapy (DOT).