Discrepancy in Vancomycin AUC/MIC Ratio Targeted Attainment Based upon the Susceptibility Testing in Staphylococcus...

Area of Science:

• Clinical Microbiology
• Pharmacokinetics and Pharmacodynamics
• Infectious Diseases

Background:

• Accurate vancomycin minimum inhibitory concentration (MIC) determination is crucial for optimizing treatment of Staphylococcus aureus infections.
• Discrepancies in MIC testing methods can impact therapeutic outcomes and guideline adherence.
• The Infectious Diseases Society of America (IDSA), American Society of Health System Pharmacists (ASHP), and Society of Infectious Diseases Pharmacists (SIDP) provide guidelines for vancomycin therapy.

Purpose of the Study:

• To compare vancomycin MIC values for Staphylococcus aureus determined by the Vitek 2 automated system and the E-test method.
• To evaluate the impact of differing MIC results on achieving pharmacokinetic/pharmacodynamic (PK/PD) targets in S. aureus bloodstream infections.

Main Methods:

• Retrospective analysis of vancomycin MICs for S. aureus bloodstream isolates using Vitek 2 and E-test.
• Calculation of target attainment based on area under the serum concentration time curve (AUC) to MIC ratio (AUC/MIC).
• Statistical comparison of target attainment rates between the two MIC testing methods.

Main Results:

• A statistically significant difference was observed in vancomycin MICs between Vitek 2 and E-test (p < 0.0001).
• Using Vitek 2 MICs, nearly 100% target attainment was achieved at an AUC threshold of 400 mg∙h/L, compared to only 40% with E-test MICs.
• An AUC of 450 mg∙h/L or greater was necessary to achieve 100% target attainment regardless of the MIC testing method.

Conclusions:

• The choice of vancomycin MIC testing method (Vitek 2 vs. E-test) significantly influences the assessment of target attainment in S. aureus bloodstream infections.
• E-test may underestimate vancomycin susceptibility, potentially leading to suboptimal dosing strategies.
• Clinical laboratories and clinicians should be aware of method-dependent discrepancies in vancomycin MICs to ensure effective patient management.