Antimicrobial susceptibility testing of Acinetobacter spp. by NCCLS broth microdilution and disk diffusion methods

Area of Science:

• Clinical Microbiology
• Antimicrobial Resistance
• Infectious Diseases

Background:

• Broth microdilution (BMD) and disk diffusion (DD) are standard methods for antimicrobial susceptibility testing.
• Limited comparative data exist for Acinetobacter spp. using these methods.

Purpose of the Study:

• To compare the categorical agreement of BMD and DD for Acinetobacter spp. susceptibility testing.
• To identify discrepancies and potential interpretation issues between BMD and DD.

Main Methods:

• Tested 196 Acinetobacter spp. isolates using BMD and DD with various antimicrobial agents.
• Evaluated error rates (very major, major, minor) between the two methods.
• Assessed BMD interpretation challenges, including subtle growth patterns.
• Collaborated with six external laboratories to confirm BMD interpretation issues.

Main Results:

• High agreement between BMD and DD for amikacin, ciprofloxacin, gentamicin, imipenem, levofloxacin, meropenem, tobramycin, and trimethoprim-sulfamethoxazole.
• Frequent very major errors with beta-lactams and beta-lactam-inhibitor combinations using BMD.
• Significant interpretation difficulties and high error rates for BMD with beta-lactams and tetracyclines.
• External laboratory testing confirmed BMD interpretation variability for cefepime.

Conclusions:

• BMD and DD show good agreement for several antibiotics against Acinetobacter spp.
• BMD poses interpretation challenges, particularly with beta-lactams, leading to unacceptable error rates.
• Clinical laboratories must be aware of potential discrepancies between BMD and DD results.