So far we have considered why measurement of antibiotic use is a cornerstone of any antimicrobial stewardship programme and in particular why measuring antibiotic use can help to improve the quality of prescribing.
Healthcare staff involved in prescribing, supplying or administering antibiotics used in routine clinical practice will be familiar with the doses of these medicines. However, within an antimicrobial stewardship programme, when it comes to measuring and expressing antibiotic use in numerical terms (remembering Kelvin’s wise words!) a standardised measure is required.
The most common, standardised measure for antibiotic use is DDDs. The World Health Organisation (WHO) have assigned DDDs to antibiotics
The basic definition for a defined daily dose is the assumed average maintenance dose per day for a drug used in its main indication in adults.
So in simple terms, the DDD is the amount of drug that a typical adult patient will receive each day for treatment of an infection.
It must be emphasised that the DDD is a technical measurement – a standardised dose used to evaluate drug use. DDDs are not clinical doses.
The DDD calculated by the WHO is often a compromise based upon information they have on doses used in a number of different countries and it often differs from those doses recommended for clinical use because the doses for individual patients will be based on individual patient characteristics, such as age, weight, and pharmacokinetic considerations, such as renal function.
In general terms, the DDDs for antibiotics are assigned according to their use in infections of moderate severity. However, for some severe infections the DDDs are assigned accordingly.
So what is the value of DDDs to a stewardship programme? In step 2.21
of Week 2 one of the essential activities listed for all hospitals was:
“Monitoring performance of antimicrobial prescribing by collecting and reporting unit or ward-specific use data, auditing antimicrobial use, and using quality use of medicines indicators”.
DDDs can be used for this monitoring especially to illustrate trends of antibiotic use over time (use going up or down for example) in a ward, hospital or group of hospitals. This is called ‘surveillance of antibiotic use’. DDD measurements may be undertaken on a monthly or quarterly basis depending on the setting and the antibiotics included.
To calculate the DDDs the total number of grams of each antibiotic used in a ward (or whole hospital) during a defined period is divided by the WHO assigned DDD value for that antibiotic. In the next step you will see a worked example of this and in Week 6 you will see further examples of how DDDs are used in a large number of hospitals in South Africa to measure antibiotic use.
In summary, front line clinicians are unlikely to use DDDs as they are not useful for measuring or informing the clinical care of individual patients.
DDDs are, however, a very useful and commonly used standardised metric to measure antibiotic use in a ward or hospital(s). Antibiotic use expressed in DDDs enables comparison of patterns of antibiotic use over time, between locations and after improvement interventions, thereby identifying areas for further investigation using audit and quality improvement methods.