EVALUATION OF METHODS IN DETECTING VANCOMYCIN MIC AMONG MRSA ISOLATES AND THE CHANGES IN ACCURACY RELATED TO DIFFERENT MIC VALUES

INTRODUCTION: Methicillin-ResistantStaphylococcus aureus (MRSA) presenting reducedsusceptibility to vancomycin has been associated to therapeutic failure. Somemethods used by clinical laboratories may not be sufficiently accurate to detectthis phenotype, compromising results and the outcome of the patient.OBJECTIVES: To evaluate the performance of methods in thedetection of vancomycin MIC values among clinical isolates of MRSA.MATERIAL AND METHODS: The Vancomycin Minimal InhibitoryConcentration was determined for 75 MRSA isolates from inpatients of Mãe deDeus Hospital, Porto Alegre, Brazil. The broth microdilution (BM) wasused as the gold-standard technique, as well as the following methods:E-test^® strips (BioMérieux),M.I.C.E^® strips (Oxoid), PROBAC^®commercial panel and the automated system MicroScan^®(Siemens). Besides, the agar screening test was carried out with 3µg/mL of vancomycin.RESULTS: All isolates presented MIC ≤ 2µg/mL for BM. E-test^® had higher concordance(40%) in terms of global agreement with the gold standard, andthere was not statistical difference among E-test^® and brothmicrodilution results. PROBAC^® panels presented MICs, ingeneral, lower than the gold-standard panels (58.66% majorerrors), while M.I.C.E.^® MICs were higher (67.99%minor errors).CONCLUSIONS: For the population of MRSA in question,E-test^® presented the best performance, although with aheterogeneous accuracy, depending on MIC values.