At our institution age-stratified antibiograms for E. coli, S. aureus, and S. pneumoniae were significantly different from the institution-wide, cumulative antibiogram. Within age groups, susceptibility varied by IP and OP location at time of specimen collection for E. coli but not for S. aureus or S. pneumoniae.
We observed, as in previous studies, that E. coli and S. aureus isolates from children were the least drug resistant, while those from patients > 65 years were the most drug resistant (Boggan et al. 2012; David et al. 2006). However, this trend was obscured by the institution-wide antibiogram which reported average values, thus overestimating resistance in pediatric isolates and underestimating resistance in isolates from the elderly. A high prevalence of E. coli and S. aureus drug resistance in the elderly is well-documented and most likely reflects greater comorbidities, hospitalizations, and antimicrobial exposure among older patients (David et al. 2006; Swami et al. 2012). In contrast, for S. pneumoniae, pediatric isolates were more drug resistant than adult isolates. The greater drug resistance among pediatric S.pneumoniae isolates is probably due to the high utilization of penicillins in children and the likelihood that the S. pneumoniae isolates used to create the antibiogram reflected complicated or refractory infections, since S. pneumoniae is not routinely cultured in uncomplicated otitis media or pneumonia.
Clinician reliance on institution-wide antibiograms that do not accurately reflect susceptibility rates in certain patient groups might lead to inappropriate empiric antibiotic prescribing. Overestimating resistance in pediatric S. aureus or E. coli could lead to prescribing of unnecessarily broad antibiotics in children which, in turn, can lead to increasingly drug resistant pathogens and C. difficile infections. For example, we have observed that local providers avoid clindamycin for treatment of pediatric skin and soft tissue infections because the cumulative antibiogram reports high rates of clindamycin resistance. An age-stratified analysis revealed that in our geographic region, clindamycin susceptibility among pediatric S. aureus isolates is above 80% and this drug remains an important therapeutic option for treatment of skin and soft tissue infections, including MRSA, in children but not in older adults. Similarly, providers in our community have followed national trends (Copp & Hersh 2011) by increasingly prescribing third generation cephalosporins for empiric treatment of pediatric urinary tract infections. Prescribing of fewer broad - spectrum agents could be encouraged by creation and dissemination of a pediatric antibiogram demonstrating E. coli with high susceptibility to narrow - spectrum cephalosporins. Boggan et al. recently reported that effective antibiotic prescribing among pediatricians, as measured through responses to clinical vignettes, improved when pediatric-specific antibiograms were provided (Boggan et al. 2012).
For elderly patients, availability of age-specific antibiograms that do not underestimate drug resistance is also likely to enhance appropriate antibiotic selection, which in turn, can optimize outcomes (Ibrahim et al. 2000). We recently determined that in our county, a quarter of elderly patients with fluoroquinolone-resistant E. coli infections received ineffective empiric therapy with a fluoroquinolone and had persistent or recurrent infections, likely due to lack of awareness among providers about local resistance rates in this age group. Furthermore, because residents of long-term care facilities are at high risk for colonization or infection with multidrug-resistant organisms, some have advocated creation of antibiograms specific for long-term care facility residents (Philippe et al. 2011). However, because many long-term care facilities lack resources to create their own antibiograms, a more practical option may be for providers to rely on hospital-based age-stratified antibiograms, as we have created.
Within age groups, we noted clinically significant differences in susceptibility of IP and OP isolates of E. coli but not of S. aureus or S. pneumoniae. E. coli OP isolates were generally more drug susceptible than IP isolates, especially among children. In contrast, for S. aureus, differences in susceptibilities between IP and OP isolates from children as well as adults were not statistically significant. These findings suggest that age- and location- stratification of E. coli, the most common pathogen isolated in children and in urinary tract infections in general, might be a valuable tool to guide empiric antibiotic selection for management of pediatric urinary tract infections.
This study has several limitations. Since it was laboratory-based, we did not have associated clinical history and could not determine if cultures represented infection or colonization, or were community-associated vs. healthcare-associated isolates. Although we eliminated duplicate isolates, our sample was also likely biased toward patients with complicated or refractory infections and prior antibiotic exposure, since such patients have cultures sent more frequently than patients with uncomplicated infections. Thus, the isolates in our collection may be more drug-resistant than those in the general population. Lastly, our susceptibility data reflect local epidemiology and may not be generalizable to other geographic regions or institutions. Despite these limitations, we demonstrate that creation of age and location-stratified antibiograms is feasible and valuable.
In conclusion, stratified antibiograms reveal age - associated differences in susceptibility of E. coli, S. aureus, and S. pneumoniae that are obscured by hospital-wide antibiograms. Further stratification of E. coli isolates by both age and IP or OP location may also be useful to clinicians who manage pediatric urinary tract infections. Although the proportion of institutions that create stratified vs. cumulative antibiograms is not clear, we believe that more facilities should create age - stratified antibiograms especially if they serve diverse patient groups (i.e. are not free-standing children’s hospitals or long-term care facilities). More research is needed to determine if improved antibiograms can be valuable stewardship tools that facilitate appropriate empiric antibiotic selection and enhance surveillance of antimicrobial resistance trends (Hebert et al. 2012; McGregor et al. 2009).
Previously presented in abstract form at the Infectious Disease Society of America meeting, 2011, Boston, MA.