Trends in Antimicrobial Consumption May Be Affected by Units of Measure

To the Editor—A recent article by Polk et al. [1] assessed the discrepancies between measures of antimicrobial consumption—specifically, defined daily doses (DDD) and days of therapy—in a sample of 130 US hospitals over a 1-year period. Those who are familiar with drug consumption studies and their methods are well aware that calculation of DDD is an attempt to estimate actual days of therapy, with the recognition that there is discrepancy but that it is likely minor overall. Others, including ourselves, have also attempted to improve on DDD measures and quantify this discrepancy [2,3,4–5]. By accessing patient records, Polk et al. [1] were able to compare the estimate with the gold standard. Although there was not a statistically significant difference observed in overall systemic antibacterial use between the 2 measures, with 6 out of 10 of the commonly used individual antibacterial agents, the difference was significant and was considered to be of major or moderate importance.