Vancomycin-intermediate resistance in Staphylococcus aureus

Registry Details

Area Health Science
Lab name Antibiotic Reference Laboratory
Lab location Kenepuru Science Centre, Porirua
Test name Vancomycin-intermediate resistance in Staphylococcus aureus
Organisms S. aureus
Requested for surveillance yes
Notice required no
Routine turnaround time 15 working days for reference specimens
Culture requirements Pure culture
Media requirements A slope of any medium that supports the growth of the organism
Pre-transport requirements Ambient temperature
Transport requirements Ambient temperature
Unacceptable sample types Clinical specimens will not be accepted.
How results reported The results and interpretation of the macromethod Etest screen, and, if performed, the results and interpretation of the population analysis profile.
Referral link Form


S. aureus isolates with vancomycin MICs of 8-16 mg/L are defined as vancomycin-intermediate S. aureus (VISA). However, often only a small proportion or subpopulation, as few as 1 in 106 cells, express the vancomycin intermediate resistance. Such isolates are referred to as heterogeneous VISA (hVISA). In standard susceptibility tests, VISA may have vancomycin MICs less than 8 mg/L.

A macromethod Etest, which uses a heavier than standard inoculum, is used as a VISA screening test. If this screen is positive, a population analysis profile (PAP) is performed to confirm the resistance and to determine whether the isolate has homogenous or heterogeneous vancomycin intermediate resistance.

Testing is recommended when there is clinical suspicion of VISA due to an apparent failure of vancomycin treatment.