An E-test strip for vancomycin is placed on a Mueller-Hinton agar plate inoculated with a Staphylococcus aureus isolate. The ellipse of inhibition intersects the strip at 3 µg/mL. According to CLSI breakpoints, how should this isolate be classified?

• A Susceptible (S): MIC ≤ 2 µg/mL is susceptible; 3 µg/mL falls into susceptible-intermediate zone
• B Susceptible: CLSI breakpoint for susceptibility is MIC ≤ 4 µg/mL for vancomycin in S. aureus
• C Resistant (R): MIC > 2 µg/mL for S. aureus is resistant by CLSI 2023
• D Intermediate (I): CLSI classifies MIC 4–8 µg/mL as intermediate (VISA range) ✓

Explanation

Per CLSI M100 (2023), CLSI vancomycin breakpoints for S. aureus are: susceptible (S) ≤2 µg/mL, intermediate (I) 4–8 µg/mL, resistant (R) ≥16 µg/mL. An MIC of 3 µg/mL technically falls between the susceptible and intermediate categories and should be repeated or interpreted with caution; by EUCAST, the susceptibility breakpoint is also ≤2 µg/mL. Thus a value of 3 µg/mL is non-susceptible and warrants clinical reassessment. VISA (vancomycin-intermediate S. aureus) is defined as MIC 4–8 µg/mL. Accurate E-test interpretation at the intersection is crucial for patient management.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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