Background Different measurements to assess antibacterial use in hospitals have been described, many of them based on the WHO Defined Daily Dose (DDD) assignments. Although WHO accepts the update of DDDs, ‘changes of DDDs should be kept to a minimum and avoided as far as possible’.

Purpose To assess the utility of WHO DDD assignments to: a) measure real-world antibacterial utilisation, and b) label prescribed doses as potentially inappropriate.

Materials and methods All courses of antibacterials for systemic use (therapeutic class J01) prescribed for therapeutic purpose in patients over 18 years-old and admitted during January 2011 were extracted from the electronic medical records. ‘Treatment days’ was obtained adding one to the difference between the starting and the ending dates. ‘DDDs used’ per patient were obtained dividing the dose actually used by the WHO DDD assignments. The ratio ‘DDD used’/'treatment days' was calculated. Outliers for this ratio were estimated by the IQR rule.

Results A total of 349 antibacterial courses were analysed comprising 1761.79 DDD, and representing 33.8 DDD/100 beds/day. Mean ratio ‘DDD used’/'treatment days' was 1.29 (SD=0.76) (range 0.16 to 7.69). This ratio was below one only for penicillins, sulfonamides and lincosamides (doses used were higher than the DDD assignments). IQR for the ‘DDD used’/'treatment days' ratio was above two for sulfonamides (IQR=2.53) and glycopeptides (IQR=2.09), identifying them as the two classes where WHO DDD assignments are more deviated from the clinical practice in our hospital. Sixteen extreme (>IQR×3) and 12 mild (>IRQ×1.5) outliers were identified, representing potential inappropriate prescriptions.

Conclusions Except for sulfonamides and glycopeptides, WHO DDD assignments could be used as an alert-generating system for potentially inappropriate antibacterial prescribed doses in our hospital by identifying the outlier prescribed doses. Further analysis is required to exclude a potential systematic inappropriate dosing for these two classes.