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