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What are defined daily doses (DDDs)?

What are DDDs?
So what are Defined Daily Doses, or DDDs, as they’re often referred to? Well, 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 a given therapeutic purpose. It must be emphasised that the DDD is a technical measurement that will necessarily differ from those doses recommended for clinical use, or indeed, prescribed to patients, because the doses for individual patients will be based on individual patient characteristics, such as age, weight, and pharmacokinetic considerations, such as renal function.
The WHO defined daily dose is often a compromise based on the information they have on the dose used in a number of different countries. So it is an average, and therefore will likely as not be different to the dose used in your own country. So what about DDDs for antibiotics? Now, in general terms, the DDDs for antibiotics are assigned according to their use in infections of moderate severity. There are, however, some infections which are severe. And in those cases, the DDDs are assigned accordingly. Some antibiotics are used in a high initial starting dose, followed by a lower maintenance dose.
And in these cases, if the total duration of treatment is beyond seven days, then it is the maintenance dose that is used to define the DDD. In cases where the treatment duration is less than seven days, the DDD is calculated as the average quantity. So the total quantity, divided by the number of days of treatment. A question that’s often asked is whether the same antibiotic, where they’re given in oral or injectable forms, has the same DDD. And in many cases, it is. So for example, amoxicillin has a DDD of 1 gram whether given by mouth or by injection. And similarly, flucloxacillin has a DDD of 2 grams, whether in oral or parenteral forms.
But on the screen now, you can see several examples of fairly commonly used antibiotics, where there is a different DDD for the oral and the parenteral forms. And in these cases, it is generally either because there is different bioavailability between the oral and the injectable forms, or where the oral are injectable forms are of use for very different types of infection. It is important that you check when using antibiotic use data to produce DDDs, whether the drug the antibiotic has a route dependent DDD. And if so, you must consider these routes separately.

In this video William Malcolm explains what defined daily doses (DDDs) are.

So far we have considered why measurement of antibiotic use is a key cornerstone of any antimicrobial stewardship programme and in particular why measuring antibiotic use can be used to improve the quality of prescribing.

This step introduces you to the main way in which antibiotic use can be measured and expressed in numerical terms (remembering Kelvin’s wise words!)

WHO – DDD – Definition and general considerations

It is worth highlighting now that DDDs are used to measure antibiotic use over time and are primarily used by antimicrobial teams to monitor trends within a ward, hospital or primary care setting. This allows the identification of areas for further investigation using audit and quality improvement methods.

Front line clinicians are unlikely to utilise DDDs as they are not useful for measuring the clinical care of individual patients.

In the next steps we will show you how to calculate DDDs and discuss their use, along with other quantitative measures, for antibiotic use. It is important to understand how these calculations are achieved although in practice a database is normally used to perform the calculations for you and details of how to access this will be provided in Step 3.20.

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Antimicrobial Stewardship: Managing Antibiotic Resistance

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