Summary of terminology for strains of S. aureus with reduced vancomycin susceptibility referred to in clinical case reportsd

Glycopeptidea susceptibility classificationBroth microdilution (μg/ml)
CLSIb prior to 2006CLSI after 2006EUCASTc
Susceptible (VSSA)≤4≤2≤2
Intermediate (VISA)8-164-8No longer included in definition
Resistant (VRSA)≥32≥16≥4
  • a Most often defined with reference to vancomycin, but some authors use “VISA” and “GISA” to indicate the presence of a class effect.

  • b NCCLS before 2005.

  • c New EUCAST breakpoints (released 20 May 2009) no longer define a VISA category and have reduced the resistant breakpoint to ≥4 μg per ml.

  • d S. aureus with reduced susceptibility (SA-RVS) is defined as follows: (i) MIC of ≥4 μg per ml (91, 306) or (ii) area under vancomycin concentration-kill curve of ≥0.9 of the AUC of type strain Mu3 (124). hVISA is defined as follows: (i) VSSA strain that upon subculture stably produces subcolonies with MICs in the VISA/VRSA range at a frequency of ≥1 × 106 according to the population analysis profile (PAP) (114); (ii) AUC ratio of ≥0.9 of the AUC of type strain Mu3, referred to as PAP/AUC (48, 118, 155); or (iii) modified high-inoculum Etest read at 48 h (193, 194).