Risk factors for acquisition of imipenem-resistant Acinetobacter baumannii: a case-control study

Risk factors for the nosocomial occurrence of imipenem-resistant Acinetobacter baumannii (IRAB) were determined. A case-control study design was used for a comparison of two groups of A. baumannii-positive patients with control patients. Nosocomial IRAB was isolated from the first group of A. baumannii-positive patients, and imipenem-susceptible A. baumannii (ISAB) was isolated from the second group. The control patients were randomly selected in a 4:1 ratio from the same medical or surgical services from which the A. baumannii-positive patients were receiving care when the isolation of IRAB occurred. Risk factors analyzed included demographic variables, comorbid conditions, variables related to hospitalization, and the antimicrobials used. IRAB was isolated from 104 patients, and ISAB was isolated from 387 patients between January and December 2000. The risk factors for IRAB were a previous intensive care unit (ICU) stay (odds ratio [OR], 21.54; 95% confidence interval [CI], 10.73 to 43.23) and prior exposure to imipenem (OR, 9.18; 95% CI, 3.99 to 21.13) or third-generation cephalosporins (OR, 2.11; 95% CI, 1.13 to 3.95). Risk factors for ISAB were a previous ICU stay (OR, 8.05; 95% CI, 5.67 to 11.44) and exposure to third-generation cephalosporins (OR, 2.07; 95% CI, 1.47 to 2.91). Our results suggest that the nosocomial occurrence of IRAB or ISAB is strongly related to an ICU stay, and IRAB occurrence may be favored by the selection pressure of imipenem.