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Vancomycin-intermediate resistance in Staphylococcus aureus

Antibiotic Reference Laboratory

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.



Health Science

Lab name

Antibiotic Reference Laboratory

Lab location

Kenepuru Science Centre, Porirua

Test name

Extended-spectrum β-lactamase (ESBL) gene PCR and sequencing


S. aureus

Requested for surveillance


Notice required


Routine turnaround time

15 working days for reference specimens

Sample 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