When introducing new antibiotic guidelines for empirical treatment of bacteremia, it is imperative to evaluate the performance of the new guideline. We examined the utility of administrative data to evaluate the effect of new antibiotic guidelines and the prognostic impact of appropriate empirical treatment. We categorized 2,008 adult patients diagnosed with bacteremia between 2010 and 2012 according to whether they received cephalosporins or fluoroquinolones (old regimen) or not (new regimen). We used administrative data to extract individual level data on mortality, readmission, and appropriateness of treatment, and computed adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) for 30-day mortality and post-discharge readmission by regimen and appropriateness of treatment. In total, 945 (47.1%) were treated by the old regimen and 1,063 (52.9%) by the new. The median length of stay (8 days) did not differ by regimen and neither did the proportion of those receiving appropriate empirical treatment (84.1% vs. 85.5%). However, fewer patients with the new regimen were admitted to the intensive care unit (ICU; 3.8% vs. 12.0%) and they had lower 30-day mortality (16.4% vs. 23.4%). The adjusted 30-day mortality HR for appropriate versus inappropriate treatment was 0.79 (95% CI 0.62-1.01) and 0.83 (95% CI 0.66-1.05) for the new versus the old regimen. The HR for 30-day readmission for appropriate versus inappropriate treatment was 0.91 (95% CI 0.73-1.13) and 1.05 (95% CI 0.87-1.25) for the new versus the old regimen. This study demonstrates that administrative data can be useful for evaluating the effect and quality of new bacteremia treatment guidelines.