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lunes, 22 de febrero de 2010

Asesora de Microbiología

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Siemens Healthcare Diagnostics Inc. c 2010 Siemens Technical Support Bulletin 175 MicroScan® Dried Gram Positive Panels 2010-01

Staphylococcus aureus with Reduced Susceptibility to Vancomycin

The detection of Staphylococcus aureus with reduced susceptibility to vancomycin is a critical concern for clinical microbiology laboratories. It became apparent with the emergence of S. aureus with reduced susceptibility to vancomycin (VISA) and those S. aureus truly resistant to vancomycin (VRSA) that the original Clinical and Laboratory Standards Institute (CLSI) vancomycin breakpoints (≤4 Susceptible, 8-16 Intermediate, ≥32 Resistant) were no longer valid. In January 2006, CLSI published new S. aureus vancomycin breakpoints (≤2 Susceptible, 4-8 Intermediate, ≥16 Resistant).1 There still remains some controversy about the new breakpoints, with some believing that they are still too high and that isolates with vancomycin MICs of 2μg/ml should not be considered fully susceptible. While Pharmacokinetic/ Pharmocodynamic (PK/PD) analysis, and some clinical studies, have indicated that strains having an MIC of 2μg/ml may be less responsive to vancomycin therapy than strains with lower MIC values, the Infectious Disease Society of America (IDSA) states that the decision to switch from vancomycin to an alternative antimicrobial agent should not be based on the vancomycin MIC alone and that patients should be monitored for response to therapy. Alternative therapy should be considered for S. aureus if the vancomycin MIC is ≥4μg/ml, but may be considered if the MIC is 2μg/ml and the patient does not respond to therapy.2 Normal distribution of vancomycin MICs in the general population shows an overwhelming mode of 1μg/ml when tested with a frozen broth microdilution panel made according to CLSI methodology.3,4 Additionally, the EUCAST (European Committee on Antimicrobial Susceptibility Testing) wildtype distribution also shows a mode of 1μg/ml, although testing methods include a combination of reference and commercial systems. Given the expected variation of antimicrobial susceptibility test results of plus or minus one doubling dilution, it is not surprising that MICs of 2μg/ml are being seen in clinical laboratories. Accurate detection of reduced susceptibility to vancomycin is further complicated by recent reports of variable results depending on the MIC testing method.5,6,7 Commercial Antimicrobial Susceptibility Test (AST) manufacturers, including Siemens Healthcare Diagnostics, reformulated vancomycin on diagnostic test systems to detect emerging resistance in Staphylococcus species. However, not all platforms (e.g. VITEK Legacy [bioMerieux, Durham, NC]) were updated, which could be a source of variability in some studies. Some commercial AST systems, including MicroScanR Dried Gram Positive Panels and ETEST strips (bioMerieux, Durham, NC) have been reported to have elevated, yet still susceptible, vancomycin MICs as compared to those obtained with the CLSI reference method.3,5,6,7 Siemens recently investigated reports received from customers of increased incidence of elevated vancomycin MICs with S. aureus on MicroScan Dried Gram Positive Panels. In most cases the vancomycin MICs were reported to be 2μg/ml, still within the CLSI susceptible interpretive category. In one internal investigation, 19 S. aureus isolates reported by a customer to give vancomycin MICs of 2μg/ml on MicroScan Dried Gram Positive Panels were evaluated. The isolates were tested on MicroScanR panels (inoculated using both the turbidity standard method and the Prompt™ Inoculation System), on a frozen reference broth microdilution panel prepared according to CLSI specifications and by the ETEST method (using an inoculum equivalent to a 0.5 McFarland standard).7 The MicroScan panels were read manually, and were also read with the WalkAwayR System. Results are shown in Table 1 below. Both the mode and average MIC of these 19 isolates on frozen reference panels was 1μg/ml. Essential (+/- 1 doubling dilution) and interpretive categorical agreement between results obtained with the MicroScan panels and the reference panels was 100%, with both WalkAway and manual reads, when using the turbidity standard method of inoculation for MicroScan panels. The mode of these isolates using the turbidity method

lunes, 22 de febrero de 2010

Asesora de Microbiología

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