Antibiotic resistance: location, location, location.

Antibiotic resistance surveys are published widely, citing percentage resistance rates, sometimes for vast transcontinental regions. Such data seem straightforward, but when one drills deeper, great complexity emerges. Rates for methicillin resistance among Staphylococcus aureus from bacteraemias vary from <1% to 50% among European countries, and vary greatly among both hospitals and hospital units. Methicillin-resistant S. aureus (MRSA) resistance rates are typically higher for tertiary-care hospitals and intensive care units than in general hospitals and wards, and lowest in single specialist centres. The likelihood of resistance also varies according to patient characteristics: those patients from nursing homes and with underlying disease, recent antibiotic treatment and hospitalisation are more likely to harbour resistant pathogens. Percentage rates themselves also may be misleading; they may be high only because the denominator is small or inaccurate; i.e., resistance may be common but the pathogen rare. Measures of disease burden-cases per 1000 bed-days or per 10(5) individuals-overcome this deficiency but are harder to collect, influenced by case mix, and associated with other problems: how to count part days or infections acquired elsewhere; most important, are all cases captured? National or international resistance statistics may illustrate trends and provide benchmarks, but for patient management, good local data are essential. Which units are most affected? Are the resistant infections locally acquired or imported with transferred patients? Are the resistant isolates clonal, indicating cross-infection, or diverse, indicating repeated selection or reflecting antibiotic policy? Unless these aspects of infection are considered, interventions to reduce resistance may be misdirected.