The need to assay the real MIC when making the decision to eradicate Staphylococcus aureus with vancomycin.

Purpose: The aim of the study was a comparison of the MIC (minimal inhibitory concentration) evaluated in the automatic system Vitek 2 and the real MIC of vancomycin by the Etest method for S. aureus strains isolated from clinical materials.

Material And Methods: Over a twelve-month study period we compared the results obtained with two commercial methods - the automatic system VITEK 2 and the real MIC by Etest - for 359 strains of S. aureus isolated from clinical materials.

Results: Most of the strains of S. aureus were cultured from wounds (84), the ear (60) and nose (42). MSSA (methicillin-sensitive Staphylococcus aureus) was isolated in 342 cases and MRSA (methicillin-resistant Staphylococcus aureus) in 17 cases. The test with the Vitek automatic method showed that vancomycin had MIC values of ≤1.0 μg/ml in more than 96% and 2.0 μg/ml in over 3% of cases. Using the Etest technique MIC ≤ 1.0 μg/ml was obtained in only 16.4% of cases and values of >1.0 μg/ml in 83.6% of cases.

Discussion: In view of such big differences between the MIC values obtained with the two methods the authors suggest that the Etest method of assaying the real MIC is more useful than the automatic method.