Unit of measurement:(defined daily dose, DDD)
The World Health Organization Collaborating Center of Drug Statistics Methodology defined the DDD as the assumed average maintenance dose per day for a drug used in its main indication in adults . It is a theoretical dose, a compromise between the dosage recommendations of various indications, fixed and regularly updated by the WHO. The DDD system measures the quantity of used drugs, independently of the dosage and the package size, which facilitates the comparisons of consumption.
The quantities in grams of antibiotics were converted in DDD and then expressed in DDD per 1000 inhabitants a day. They are recommended by the WHO as the unit of measurement for drug utilization studies in the ambulatory care setting .
In outpatient care, the total consumption of antibacterial agents for systemic use (ATC group J01) was 10.7 DDDs per 1000 inhabitants per day in 2017 (11.1 in 2016 and 11.3 in 2015, Figure 1).
Figure 1. Antibiotic consumption (ATC group J01) expressed in DDD per 1000 inhabitants and per day in the outpatient setting, Switzerland.
The most commonly used class of antibiotics was the penicillins (ATC group J01C), followed by the macrolides, lincosamides and streptogramins (ATC group J01F), tetracyclines (ATC group J01A) and fluoroquinolones (ATC group J01MA). The relative consumption of fluoroquinolones and penicillins associated with beta-lactamase inhibitors was relatively high in comparison with countries participating in the European Surveillance of Antimicrobial Consumption Network (ESAC-Net) .
 World Health Organization Collaboration Center for Drug Statistics Methodology. ATC index with DDDs. Oslo : World Health Organization, 2005.
 Dukes MN. Drug utilization studies. Methods and uses. WHO Reg Publ Eur Ser. 1993 ; 45 :1-4.  European Centre for Disease Prevention and Control. Antimicrobial consumption. In: ECDC. Annual Epidemiological Report for 2016. Dowloadable tables. Stockholm: ECDC; 2018. Available from: https://ecdc.europa.eu/en/publications-data/downloadable-tables-antimicrobial-consumption-annual-epidemiological-report-2016