Acinetobacter baumannii 2002–2008: Increase of Carbapenem-Associated Multiclass Resistance in the United States

Abstract

The study consisted of data for 55,330 U.S. Acinetobacter baumannii isolates from The Surveillance Network^® database for the period 2002–2008. Risk factors were time, age, sex, census region, location (Ward or ICU), and isolate source. Antimicrobial susceptibility data were available for carbapenems, cephalosporins, aminoglycosides, fluoroquinolones, and β-lactam/β-lactamase inhibitor combinations. Multiclass resistance was defined as nonsusceptibility to carbapenems and two or more additional classes. Odds of resistance were obtained using a logistic regression model with cubic splines. Carbapenem-associated multiclass resistance has had a 3.7-fold (95% confidence interval [CI] 3.4–4.3) increase from 20.6% in 2002 to 49.2% in 2008. Among blood isolates the increase was by 2.2 times (95% CI 1.7–2.9). Subjects <18 years old had significantly (p < 0.001) lower rates in 2002 (6.9%) than those 65 years or older (21.5%), but by 2008 this difference diminished as rates increased to 44.2% and 54.2%, respectively. A similar divergence was also observed between ICU and Ward, with no differences in 2002, whereas in 2008 ICU isolates had significantly higher rates (55.2%, 95% CI 53.6%–56.9%) than Ward isolates (45.6%, 95% CI 44.2%–47.0%). Over half of all A. baumannii–resistant isolates were carbapenem and multiclass resistant in 2008. Rates among subjects <18 years old have increased faster than those of the elderly, and in the ICU as compared to Ward.