The E-test (epsilometer test) is used to determine MIC of vancomycin against a Staphylococcus aureus isolate. The MIC read is 4 µg/mL. According to CLSI breakpoints for S. aureus, how is this isolate classified, and what is the clinical concern?
- A Susceptible; MIC ≤2 µg/mL is susceptible, but 4 µg/mL is within the susceptible range — no clinical concern
- B Vancomycin-intermediate S. aureus (VISA); CLSI breakpoint: susceptible ≤2 µg/mL, intermediate 4–8 µg/mL; these strains show reduced clinical response to vancomycin even when MIC is 4 µg/mL ✓
- C Vancomycin-resistant S. aureus (VRSA); MIC ≥16 µg/mL defines VRSA; MIC 4 µg/mL is vancomycin-resistant
- D The MIC cannot be read accurately by E-test at this range; MGIT broth microdilution must be performed
Explanation
Per CLSI criteria for S. aureus: vancomycin susceptible ≤2 µg/mL; VISA (vancomycin-intermediate S. aureus) = 4–8 µg/mL; VRSA (vancomycin-resistant) ≥16 µg/mL. An MIC of 4 µg/mL classifies the isolate as VISA. The clinical concern is that even within the 'intermediate' range, vancomycin pharmacokinetic/pharmacodynamic targets (AUC/MIC ≥400) may be difficult to achieve without toxicity. VISA strains have thickened cell walls (reduced vancomycin penetration) as the resistance mechanism. These strains should prompt consideration of alternative agents (linezolid, daptomycin, telavancin, ceftaroline for MRSA/VISA). E-test is accepted as a valid MIC method for vancomycin against staphylococci.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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