When antibiograms mislead

Whether in our coat pocket or e-device, most of us carry an antibiogram whenever we are seeing patients. The antibiogram is meant to help guide empiric antimicrobial therapy, and usually provides “percent susceptible” for most common bug/drug combinations. However, too many antibiogram users do not understand the limitations of using these aggregate data to treat individual patients. Most understand that hospital-wide antibiograms aren’t applicable across every unit, and formulating unit-specific antibiograms is now common practice.

A more concerning issue, though, is that current antibiogram guidelines recommend including only the first isolate of a given species from each patient, excluding subsequent isolates. While this approach prevents a single patient from having undue influence on the aggregate data, it has the effect of ignoring the risk of emerging resistance during prolonged hospitalizations. In this month’s issue of ICHE, Duke investigators demonstrate the consequences that this approach may have. Read this study for yourself, but the short version: antibiograms lose their predictive utility for Pseudomonas aeruginosa susceptibility by about day 10 of hospitalization for most important anti-pseudomonal agents.