While the prevalence of MRSA among all Staph aureus isolates is increasing, in vitro evidence of growing resistance of MRSA isolates to vancomycin is also increasing. In vitro susceptibility testing for any organism and antimicrobial is expressed as minimal inhibitory concentration (MIC). MIC is the lowest concentration of an antimicrobial that will inhibit growth of an organism. That is, the higher the MIC for a given antimicrobial and organism, the greater concentration of drug required to produce killing. Vancomycin MIC's for MRSA are rising, a phenomenon referred to as "MIC creep"18. Wang et al reported mean vancomycin MIC's for MRSA at UCLA to be ≤ 0.5ug/ml in 2000 and 1mg/dl in 2004 and the prevalence of isolates with MIC=1ug/ml to be 70% in 2004 vs 20% in 2000. MIC's for a given isolate in a given patient may change over time as well. Webster et al reported a MRSA isolate with an MIC changing from ≤ 1 to 4ug/ml over several months, demonstrating the ability of MRSA to acquire or alter its resistance patterns19. S. aureus isolates for which vancomycin MICs are 2-8 μg/mL are classified as vancomycin-intermediate, and isolates for which vancomycin MICs are ≥16 μg/mL are classified as vancomycin-resistant by the Centers for Disease Control20. Clinical outcome with vancomycin therapy now appears to correlate inversely with MIC's above 0.5ug/ml, so that as MIC's increase, clinical outcome worsens, suggesting that although MIC's ≤ 2ug/ml are technically reported as "Susceptible", the clinical efficacy of vancomycin may vary between isolates with MIC's of 0.5ug/ml and isolates with MIC's of 1 or 2ug/ml.