Vancomycin MIC creep in Staphylococcus aureus collected over a period of four years in a tertiary care centre

Title Vancomycin MIC creep in Staphylococcus aureus collected over a period of four years in a tertiary care centre Introduction S.aureus is one of the wellestablished and successful pathogens worldwide causing pyogenic as well as toxin mediated diseases. Methicillin resistant Staphylococcus aureus (MRSA) is currently one of the important pathogens responsible for many hospital acquired infections, especially in the Western population. Vancomyin is the drug of choice for MRSA infections. Complete vancomycin resistance of S.aureus has been established only in few circumstances. However,there are studies and meta-analysis which has shown that MRSA strains with higher MICs to vancomycin are more difficult to treat even though the MIC falls in the susceptible range. Retrospective analyses of vancomycin MIC over years in many institutes have observed a phenomenon where the numbers of isolates with higher MICs have been rising over the years. This phenomenon has been termed MIC creep. Aim To determine the existence of vancomycin MIC creep in S.aureus in our teritiary care centre Methods Consecutive clinical MRSA isolates from blood, sputum, pus, CSF,urine and body fluids of patients admitted to the hospital from January 2010 to December 2013 were examined. Cefoxitin resistance was determined using Kirby Bauer disc diffusion and interpretation was based on CLSI guidelines. Vancomycin MIC was determined using agar dilution technique. The geometric mean MIC was calculated using STATA IC 10.1 software.Results A total of 6641 isolates of MRSA was included in the study. The percentage rise of MIC over four years show an initial increase in isolates with MIC of 1 microgram per ml in the year 2011 as compared to 2010. However, there was an isolated significant fall in the percentages of isolates with an MIC of one in the past two years (p 0.01). No consistent trend was observed in the mean MICs over four years.Conclusion The geometric mean