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Comparison of two commercially available vancomycin discs for interpreting susceptibility patterns among gram positive cocci: Ongoing challenging journey in resource poor settings

Antibiotic discs made by different manufacturers may vary in their working potencies for various reasons and this can affect implementing standard interpretative criteria for reporting susceptibility patterns for certain key antibiotics. The present study was aimed at finding out whether zone diameters produced by certain important Gram positive cocci using two makes of vancomycin discs interfere with interpretation of susceptibility patterns and thus clinical management of patients affected when tested by the widely followed Kirby Bauer disc diffusion method in routine Microbiology practice in the scenario of resource poor settings. A random sample of 37 clinical isolates of Gram positive cocci including staphylococci, enterococci and streptococci was tested for susceptibility to vancomycin by Kirby Bauer disc diffusion method using discs of two makes and the effect of available standard breakpoint interpretative criteria for reporting susceptibility was analyzed. The average vancomycin zone diameter for the make A (17.972mm) was higher than the average zone diameter for the make B (16.013mm) and the difference was significant at p=0.0004 by student t test. The overall variation in decisive interpretations for the isolates for the two makes was 28 %, more variant for enterococci (50%) than staphylococci (15%). Appropriate makes of vancomycin discs must be chosen as part of robust internal quality control mechanisms to minimize errors and dilemmas in interpretation of susceptibility patterns as per standard guidelines against commonly encountered gram positive cocci. All diagnostic Microbiology labs must adapt themselves to the growing need for standardization for providing better healthcare in the community. | Comparison of two commercially available vancomycin discs for interpreting susceptibility patterns among gram positive cocci: Ongoing challenging journey in resource poor settings