Parallel analysis of individual and aggregated data on antibiotic exposureand resistance in gram-negative bacilli

To evaluate the potential bias of analyzing aggregated data, we separately examined antibiotic exposure and resistance data for 35,423 patients admitt ed to a university hospital in Utah, from both an individual-patient perspe ctive and group-level perspective. From 1994 through 1998, use of defined d aily doses (per 1000 patient-days) of fluoroquinolones, third-generation ce phalosporins, ampicillin-sulbactam, and imipenem increased by 82%, 38%, and 99%, and decreased by 38%, respectively, whereas group-level resistance ra tes of Enterobacteriaceae or Pseudomonas species changed only minimally. Ho wever, in individual-patient-level analyses performed by multivariable prop ortional hazards regression, exposure to a fluoroquinolone, third-generatio n cephalosporin, ampicillin-sulbactam, or imipenem was a strong risk factor for resistance to fluoroquinolones (adjusted hazard ratio [AHR], 4.0; P<.0 01), third-generation cephalosporins (AHR, 3.5; P<.001), ampicillin-sulbact am (AHR, 2.3; P = .008), or imipenem (AHR, 5.7; P < .001), respectively. Th us, group-level and individual-patient-level analyses of antibiotic-use-ver sus-susceptibility relations yielded divergent results. Multicenter studies should include individual-patient-level data to elucidate more fully the r elation between antibiotic exposure and resistance.