Antimicrobial stewardship lessons from mupirocin use and resistance in methicillin-resitant Staphylococcus Aureus

Background. The quantitative relationship between antimicrobial agent consumption and rise or fall of antibiotic resistance has rarely been studied. We began all admission surveillance testing for methicillin-resistant Staphylococcus aureus (MRSA) in August 2005 with subsequent contact isolation and decolonization using nasally applied mupirocin ointment for those colonized. In October 2012, we discontinued decolonization of medical (nonsurgical service) patients. Methods. We conducted a retrospective study from 2007 through 2014 of 445 680 patients; 35 235 were assessed because of mupirocin therapy and positive test results for MRSA. We collected data on those patients receiving 2% mupirocin ointment for decolonization to determine the defined daily doses (DDDs). A nonparametric regression technique was used to quantitate the effect of mupirocin consumption on drug resistance in MRSA. Results. Using regressive modeling, we found that, when consumption was consistently > 25 DDD/1000 patient-days, there was a statistically significant increase in mupirocin resistance with a correlating positive rate of change. When consumption was ≤25 DDD/1000 patient-days, there was a statistically significant decrease in mupirocin resistance with a correlating negative rate of change. The scatter plot of fitted versus observed mupirocin resistance values showed an R² value of 0.89-a high correlation between mupirocin use and resistance. Conclusions. Use of the antimicrobial agent mupirocin for decolonization had a threshold of approximately 25 DDD/1000 patient-days that separated a rise and fall of resistance within the acute-care setting. This has implications for how widely mupirocin can be used for decolonization, as well as for setting consumption thresholds when prescribing antimicrobials as part of stewardship programs.