Background: Problematic vancomycin-susceptible Staphylococcus aureus (PVSSA) having minimum inhibitory concentrations (MICs) in the 1.5-2.0 µg/mL range associate with elevated rates of therapeutic failure, even among MSSA subtypes treated exclusively with β-lactams. This suggests that poorer clinical outcomes are not only due to glycopeptide MIC. Increasing PVSSA incidence and decreasing vancomycin use in the MHS were demonstrated previously. Here, we asked if rising PVSSA incidence levels positively correlated with β-lactam usage, differed relative to methicillin susceptibility, or differed by U.S. geographic region.
Methods: Prescriptions per 1,000 inpatient encounters were recorded from Jan 2010 through Dec 2014 for cefazolin, cephalexin and nafcillin. The ratio of PVSSA relative to all VSSA seen during this observation period was also analyzed. Seasonal-trend decomposition (STL) extracted trends from aggregate time series data, and Spearman correlation measured the association between trend lines for each β-lactam relative to PVSSA incidence. The Cochran-Mantel-Haenszel (CMH) test assessed dependencies between methicillin sensitivity (MRSA/MSSA) and PVSSA incidence (α = 0.05). Ordinary least squares lines were fit to PVSSA levels in the North, South and West.
Results : A significant upward trend was seen in PVSSA incidence, and downward trends in usage were observed for all three β-lactams (Figure 1). Correlations between each antimicrobial agent relative to PVSSA incidence were negative. The CMH test suggested no difference in PVSSA status vis-ŕ-vis MRSA/MSSA classification (Figure 2). The North exhibited the highest PVSSA incidence during 2010, the South during 2012, and the West during 2014 (Figure 3). PVSSA incidence declined in the North and increased in the South and West from 2010 to 2014.
Conclusion : Escalating levels of PVSSA were not associated with β-lactam or glycopeptide prescriptions in this large, geographically dispersed managed care network. PVSSA incidence did not correlate with MRSA/MSSA status. PVSSA incidence rose most steeply in the West. In conjunction with prior findings, these results suggest that factors other than, or in addition to, β-lactam and vancomycin use are drivers of increasing rates of PVSSA.
R. Clifford, None
E. Milburn, None
P. Waterman, None
C. Neumann, None
M. Julius, None
M. Hinkle, None
E. Lesho, None