A confounding factor in the determination of MIC's for vancomycin or other antimicrobials is the variability with which MIC's are measured. MIC's may be obtained by automated equipment or by direct visual assessment of killing by antimicrobial disks or strips on a culture plate. Automated equipment historically does not report specific vancomycin MIC's below or equal to 2ug/ml. The limitation with such equipment is that as vancomycin MIC's for MRSA have crept upward over time, we now have automated equipment that reports all MRSA isolates with vancomycin MIC's ≤ 2 as susceptible-with no distinction of whether the MIC is 0.5, 1 or 2ug/ml. Automated equipment generally reports MIC's in doubling of antimicrobial concentration. Furthermore, while the majority of the sample may contain isolates with vancomycin MIC's in the susceptible range, we now know that susceptibility may vary within a given sample. Some solitary organisms or colony-forming units (CFU's) may demonstrate a phenotype in which vancomycin MIC's are higher than the majority of the sample but their numbers are so small as to be unrecognized. This variability in MRSA resistance to vancomycin is termed heteroresistance and heteroresistance may contribute to treatment failure. Moreover, ideal methods to detect and address vancomycin heteroresistance in S. aureus (hVISA) in every institution or laboratory have yet to be established.