CDC outlines staph testing options for labs

The Centers for Disease Control and Preven tion’s recent recommendations for testing for Staphylococcus aureus with reduced susceptibility to vancomycin include the following measures:1

Strategies for selection of isolates for additional testing:

Select isolates with vancomycin minimum inhibitory concentrations (MICs) of greater than or equal to 4 µg/mL. This is based on the apparent heterogeneity of strains because organisms with MICs of greater than or equal to 4 µg/mL have subpopulations with higher MICs. Clinical treatment failures have occurred with vancomycin in infections with these isolates.

Select isolates with vancomycin MICs of greater than or equal to 8 µg/mL (based on National Committee for Clinical Laboratory Standards [NCCLS] breakpoints. NCCLS MIC breakpoints for vancomycin are: susceptible, less than or equal to 4µg/mL; intermediate, 8-16 µg/mL; and resistant, greater than or equal to 32 µg/mL).

Select all methicillin-resistant S. aureus (MRSA). All identified isolates of S. aureus with reduced susceptibility to vancomycin have been MRSA.

Select all S. aureus isolates. Because little is known about the extent of this resistance, any S. aureus potentially could have reduced susceptibility to vancomycin.

Testing and confirmation:

Primary testing of S. aureus against vancomy cin requires 24 hours of incubation time.

Disk diffusion is not an acceptable method for vancomycin susceptibility testing of S. aureus. None of the known strains of S. aureus with reduced susceptibility to vancomycin have been detected by this method.

An MIC susceptibility testing method should be used to confirm vancomycin test results.

[Editor’s note: More information about testing for vancomycin intermediate-resistant S. aureus and other resistant organisms is available on the CDC hospital infections Web site at www.cdc.gov/ncidod/ hip.htm (click on "Laboratory").]

Reference

1. Centers for Disease Control and Prevention. Laboratory capacity to detect antimicrobial resistance. MMWR 2000; 51:1,167-1,171.