Correlation between Vancomycin MIC Values and Those of Other Agents against Gram-Positive Bacteria among Patients with Bloodstream Infections Caused by Methicillin-Resistant Staphylococcus aureus

An increase in the distribution of vancomycin MIC values among methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates has been noted. It is postulated that the shift in vancomycin MIC values may be associated with a concurrent rise in the MIC values of other anti-MRSA agents. Scant data are available on the correlation between vancomycin MIC values and the MIC values of other anti-MRSA agents. This study examined the correlation between vancomycin MIC values and the MIC values of daptomycin, linezolid, tigecycline, and teicoplanin among 120 patients with bloodstream infections caused by MRSA at a tertiary care hospital between January 2005 and May 2007. For each included patient, the MIC values of the antibiotics under study were determined by the Etest method and were separated into the following two categories: day 1 (index) and post-day 1 (subsequent). For subsequent isolates, the MIC values for each antibiotic from the post-day 1 terminal isolate were used. Among the index isolates, there was a significant correlation (P value, <0.01) between the MIC values for vancomycin and daptomycin and between the MIC values for vancomycin and teicoplanin. The MIC values for daptomycin were significantly correlated with linezolid, tigecycline, and teicoplanin MIC values. Among the 48 patients with subsequent isolates, vancomycin MIC values were significantly correlated with MIC values for daptomycin, linezolid, and teicoplanin (ρ value of ≥0.38 for all comparisons). This study documented an association between vancomycin MIC values and the MIC values of other anti-MRSA antibiotics among patients with bloodstream infections caused by MRSA primarily treated with vancomycin.An increase in the distribution of vancomycin MIC values among methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates has been noted in several recent reports (3, 11, 14). This shift is a concern because a growing number of studies have shown that patients with infections caused by MRSA with vancomycin MIC values at the higher end of the Clinical and Laboratory Standards Institute (CLSI) and Food and Drug Administration (FDA) susceptibility range are less responsive to vancomycin (3, 4, 5, 7, 12, 13). Clinicians must now consider using alternative therapies for such patients. However, it is unclear whether the observed shift in the distribution of vancomycin MIC values for MRSA is associated with similar shifts in the distribution of MIC values of other anti-MRSA agents.It is postulated that the shift in vancomycin MIC values may be associated with a concurrent rise in the MIC values of other anti-MRSA agents. Scant data are available on the correlation between vancomycin MIC values and the MIC values of other anti-MRSA agents. To date, analyses have been limited to the index MRSA isolate and have primarily focused on the correlation between vancomycin and daptomycin (6, 10, 11).This study examined the correlation between vancomycin MIC values and the MIC values of daptomycin, linezolid, tigecycline, and teicoplanin among patients with bloodstream infections caused by MRSA. Given the recent reports describing the emergence of resistance during therapy, our analyses included both the index (day 1) and subsequent (post-day 1) isolates.