Survey of University Hospital Practices to Detect Increased MICs to Vancomycin (V) in Clinical Isolates of Staphylococcus aureus (SA)

Background: V is widely used for treatment of infections caused by methicillin-resistant SA.  Some investigators report the V MIC of SA is increasing in small increments, called MICcreep, and is associated with poor clinical response to V. Others suggest differences in testing methods may be responsible for MICcreep. It is unclear how hospitals are currently responding to these conflicting data.

Objective: To survey a sample of US academic teaching hospitals to determine current practices to detect MICcreep, interpretation of the results and the impact on clinical practice. 

Methods: A 12 question on-line survey was sent May 6, 2010 to contacts at 59 hospitals that participate in the University HealthSystem Consortium (UHC) Clinical Resource Manager (CRM) program. We surveyed routine susceptibility testing methods for SA to V, if confirmatory V susceptibility testing was performed, the methods used, whether this decision was related to vancomycin use, and hospital policy regarding interpretation of results and recommendations for V-alternative therapy.

Results: 42 hospitals completed the survey (71%). Routine SA susceptibility testing included Vitek (17), Microscan (n=13), Phoenix (n=8), Etest (n=2), Trek (n=1) and disc diffusion (n=1). Confirmatory testing of the V MIC for SA was routine for 23 hospitals; 19 performed no additional testing. The volume of vancomycin use in hospitals performing confirmatory tests (133±32 DOT/1000PD) was not different from hospitals that did not (127±30 DOT/1000PD). Of the 23 hospitals, confirmatory testing began from ‘always' through December 2009. Confirmatory testing for V MICs was by E-test (n=11) and macro E-test (n= 7), for all SA (n=18) or MRSA only (n=5). Confirmatory MIC testing was routine for all clinical isolates (n = 14), or was limited to requests from ID or pharmacy, or for selected culture sites such as blood (n=9). A measured V MIC that represented “concern” was reported to be ≥1.0- (n = 6), ≥1.5- (n =  4), or ≥ 2.0 mg/ml (n = 13). Hospital policy for managing such isolates ranged from “no formal policy” (n = 15) to active interventions that usually depended on clinical response, V MIC and site of infection (n=7). Alternative therapies in light of increased MICs most often included daptomycin for BSIs, linezolid for pneumonia and various therapies for SSSIs. Tigecycline and telavancin were also used. Hospitals often responded in free text that this is an area of active discussion confounded by incomplete data and plans for continued monitoring.

Conclusions: Approximately 50% of this sample of university teaching hospitals routinely confirm MICs for SA to V, most often by E-test. Interpretation of the significance of measured V MICs was variable and many respondents expressed uncertainty as to the most appropriate detection and management strategy for SA isolates with increased V MICs.