Vancomycin MICs did not creep in Staphylococcus aureus isolates from 2002 to 2006 in a setting with low vancomycin usage

Objectives The aim of this study was to evaluate MIC trends for clinical isolates of Staphylococcus aureus to vancomycin over a 5 year period (2002–06) in a hospital in Spain. Methods All clinical isolates of S. aureus (one per patient) from clinical samples of patients at Hospital Universitario de Getafe from January 2002 to December 2006 were used. MICs of vancomycin were determined by the CLSI broth microdilution procedure. For analysis of MIC trends over the 5 years, we grouped the isolates into those with MIC ≤1 mg/L [2428 methicillin-susceptible S. aureus (MSSA) and 518 methicillin-resistant S. aureus (MRSA)] and those with MIC ≥2 mg/L (MIC = 2 mg/L: 141 MSSA and 47 MRSA; MIC = 4 mg/L: 5 MSSA and 1 MRSA). MICs for the different groups in the different years were compared with the linear-trend χ2 test. Results A total of 3141 strains of S. aureus collected over the 5 year period was included in this analysis. Of these, 2574 (82%) strains were MSSA and 566 (18%) strains were MRSA. One of the 566 MRSA strains (0.18%) and 5 of the 2574 MSSA strains (0.19%) were vancomycin-intermediate (not significant). The rest were susceptible. The overall percentage of MRSA isolates with a vancomycin MIC of ≥2 mg/L was much higher than that of MSSA during the 5 year period [8.5% (48/566) versus 5.7% (146/2574); P = 0.012]. No statistically significant change in the percentage of isolates with a vancomycin MIC of ≥2 mg/L was observed over the years for MRSA (χ2 = 0.01; P = 0.91) or MSSA (χ2 = 0.08; P = 0.78). Annual consumption of parenteral vancomycin in our hospital in daily defined doses/100 stays was: 2002 (1.91), 2003 (1.63), 2004 (1.74), 2005 (2.06) and 2006 (1.64). Conclusions In a setting of low consumption of vancomycin and with a large collection of S. aureus clinical isolates, we have demonstrated the stability of vancomycin MICs over time.