Evaluation of Antimicrobial Resistance Among Gram-Negative Isolates Collected from Intensive Care Units and Reliability of Routine Disc Susceptibility Tests at a Teaching Hospital in Tehran

Antibiotic resistance of Intensive Care Unit (ICU) born aerobic gram-negative bacteria was evaluated during a six months period at a teaching hospital in Tehran, and determination of the validity of the results obtained from disc diffusion tests, using discs manufactured in Iran.

Disc susceptibility tests using Iranian and standard discs (diffusion discs available in international markets) were performed on 108 aerobic gram-negative isolates obtained from the clinical samples of patients with at least 72 hours of stay in the ICU. The Minimum Inhibitory Concentrations (MIC) was subsequently determined by collaborators not involved with the disc testing evaluation.

Acinetobacter was the most frequently isolated gram negative species (26%). High resistance rates were obtained for all antibiotics studied except for imipenem (98% sensitive). Results of disc diffusion tests performed by the Iranian discs were in moderate to strong agreement with those obtained from the standard discs. When comparing disc results with the MIC results, it was noted that the total number of very major, and minor discrepancies were approximately the same with both sets of discs. The total number of major discrepancies was higher for the Iranian discs (more false positive results). The total number of very major discrepancy rates was more than the acceptable 1.5% limits for each antibiotic tested, using both Iranian and standard discs.

High rates of resistance in aerobic gram-negative isolates studied, leaves imipenem as the only reliable agent for the empirical treatment of ICU infections. The high rates of very major discrepancies with both sets of discs show that physicians cannot rely on disc diffusion tests only, in order to guide therapy for the treatment of very serious infections in the ICUs, even if standard discs are used instead of the Iranian discs, and there is great need to establish a fast and easy way to determine the MIC values. Although it is better to repeat the study with a much larger sample size in order to make good judgment.