Activities of Tigecycline against Clinical Isolates of Acinetobacter baumannii in Taiwan: Broth Microdilution Method vs. Disk Diffusion Method

Background: The Tigecycline In-Vitro Surveillance in Taiwan (TIST), initiated in 2006, is a nationwide surveillance program designed to monitor longitudinally the in vitro activities of tigecycline against commonly encountered resistant bacteria in Taiwan. This study aims to compare the in vitro activities of tigecycline against clinical isolates of Acinetobacter baumannii by using two susceptibility methods. Methods: A total of 393 isolates of A. baumannii were collected from various sources of patients treated at 20 teaching hospitals. Minimum inhibitory concentrations (MICs) and diameters of inhibitory zone for tigecycline were determined by the broth microdilution methods and the disk diffusion method, respectively. The results were interpreted by the MIC criteria provided by U.S. FDA tigecycline susceptibility breakpoints listed for Enterobacteriaceae (S, ≤2 μg/mL; I, 4 μg/mL; R, ≥8 μg/mL) and by the disk diffusion breakpoints (S, ≤16 mm; I, 13–15 mm; R, ≥12 mm) recommended by Jones et al (J. Clin. Microbiol 2007;45:227–30). A very major error (VME) rate of < 1%, major error (MaE) rate of <5%, and a total error rate of <20% were considered acceptable. Results: Percentages of susceptible, intermediate, and resistant isolates determined by the broth microdilution method (disk diffusion method) are 81.7% (88.3%), 12.0% (8.9%), and 6.3% (2.8%), respectively. VME, MaE, minor error, and total error rates were 0.5% (2/393), 8.9% (35/393), 37.4% (147/393), and 46.8% (184/393), respectively. Conclusion: Comparison with the broth microdilution method for tigecycline against A. baumannii, the disk diffusion method tends to have a high false-susceptibility rate and have an unacceptable high VME.