Comparison of Automated and Manual Methods for Antimicrobial Susceptibility Testing

Background: Carbapenems are considered the broadest-spectrum β-lactam agents and are often required for treatment of severe hospital-acquired infections caused by multidrug-resistant Gram-negative organisms. Minimum inhibitory concentrations (MICs) are important in diagnostic laboratories to confirm resistance of microorganisms to an antimicrobial agent and also to monitor the activity of new antimicrobial agents.

Aims and Objectives: To compare the MIC obtained by Broth Microdilution method (BMD) with that of Vitek-2(automated method) for recovered isolates of Klebsiella pneumoniae.

Materials and Methods: Prospective study conducted over a period of one year. It included all isolates of Klebsiella pneumoniae recovered from blood culture of the patients. The identification and antimicrobial susceptibility was done on Vitek-2.These Isolates were subjected to Microbroth dilution method for MIC determination.

Results: Out of the 55 meropenem resistant isolates by vitek-2, 20(36.3%) had MIC of ≥256 µg/ml followed by 18(32.7%) isolates with a MIC of 128 µg/ml, followed by 11(20%) isolates with MIC of 64 µg/ml and 6(10.9%) isolates with MIC of 32 µg/ml. Also among 15 meropenem sensitive isolates by Vitek-2, 13(86.7%) had MIC of ≤0.5 µg/ml, followed by two (13.3%) isolates with MIC of 2 µg/ml. Results obtained by vitek 2 were compared with those from BMD(the reference method), which showed a 13.3% minor error rate and no major or very major error rate.

Conclusion: Overall, the Vitek 2 performance was comparable to that of BMD for testing a limited number of Klebsiella pneumoniae isolates.