Title:Comparison of Liofilchem and Etest gradient strips, and BD Phoenix, for the determination of vancomycin MIC in Staphylococcus aureus

Author:Julie A Creighton

Abstract:Introduction: Vancomcyin is the treatment of choice for serious infections caused by Gram positive organisms, with therapy optimisation based on a calculation of the AUC24/MICBMD ratio. Laboratories in New Zealand use a variety of methods to determine vancomycin MIC, including Vitek, Phoenix and Etest and Liofilchem MIC strips; however, there is a paucity of information on the performance of Liofilchem strips. This study compared Liofilchem and Etest gradient strips against Phoenix, for the determination of vancomycin MIC; also assessing variability between methods and operators to establish the reliability of reporting single dilution MIC values to clinicians.
Methods: A selection of 100 Staphylococcus aureus isolates, including 48 MRSA, were included in the study. Phoenix broth micro dilution (BMD), was performed using panel PMIC-84, and gradient strip MICs were performed using Etest and Liofilchem MIC Strips, recording single and double dilution MICs.
Results: All isolates were vancomycin susceptible, giving 100% categorical agreement. The essential agreement (EA) (MIC ±1 log2) between all methods was 97%, with Phoenix and Etest showing the highest EA at 100% and modal values of 1.0 mg/L. Phoenix and Liofilchem had the lowest EA of 97%, due to the lower modal value of 0.5 mg/L produced by Liofilchem. Absolute agreement for single dilution values between Etest and Liofilchem was very low at 14%. Reader variability for the MIC strips ranged from 57% absolute agreement (Liofilchem at single dilution values) to 89% (Etest at double dilution values).
Conclusions: This study demonstrated high EA between methods, but considerable operator and method variation between MIC gradient strips. Liofilchem tended to produce MIC values one dilution lower than both Etest and Phoenix. These results have implications in terms of MIC method variability and the capacity of the laboratory to accurately report an absolute MIC result.
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