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Journal of Clinical Microbiology, June 2009, p. 1640-1644, Vol. 47, No. 6
0095-1137/09/$08.00+0     doi:10.1128/JCM.02135-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Vancomycin MIC plus Heteroresistance and Outcome of Methicillin-Resistant Staphylococcus aureus Bacteremia: Trends over 11 Years {triangledown}

Adina C. Musta, Kathleen Riederer, Stephen Shemes, Patrick Chase, Jinson Jose, Leonard B. Johnson, and Riad Khatib*

Department of Medicine, Infectious Diseases, St. John Hospital and Medical Center, Detroit, Michigan

Received 7 November 2008/ Returned for modification 31 December 2008/ Accepted 30 March 2009

Vancomycin MICs (V-MIC) and the frequency of heteroresistant vancomycin-intermediate Staphylococcus aureus (hVISA) isolates are increasing among methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates, but their relevance remains uncertain. We compared the V-MIC (Etest) and the frequency of hVISA (Etest macromethod) for all MRSA blood isolates saved over an 11-year span and correlated the results with the clinical outcome. We tested 489 isolates: 61, 55, 187, and 186 isolates recovered in 1996-1997, 2000, 2002-2003, and 2005-2006, respectively. The V-MICs were ≤1, 1.5, 2, and 3 µg/ml for 74 (15.1%), 355 (72.6%), 50 (10.2%), and 10 (2.1%) isolates, respectively. We detected hVISA in 0/74, 48/355 (13.5%), 15/50 (30.0%), and 8/10 (80.0%) isolates with V-MICs of ≤1, 1.5, 2, and 3 µg/ml, respectively (P < 0.001). The V-MIC distribution and the hVISA frequency were stable over the 11-year period. Most patients (89.0%) received vancomycin. The mortality rate (evaluated with 285 patients for whose isolates the trough V-MIC was ≥10 µg/ml) was comparable for patients whose isolates had V-MICs of ≤1 and 1.5 µg/ml (19.4% and 27.0%, respectively; P = 0.2) but higher for patients whose isolates had V-MICs of ≥2 µg/ml (47.6%; P = 0.03). However, the impact of V-MIC and hVISA status on mortality or persistent (≥7 days) bacteremia was not substantiated by multivariate analysis. Staphylococcal chromosome cassette mec (SCCmec) typing of 261 isolates (including all hVISA isolates) revealed that 93.0% of the hVISA isolates were SCCmec type II. These findings demonstrate that the V-MIC distribution and hVISA frequencies were stable over an 11-year span. A V-MIC of ≥2 µg/ml was associated with a higher rate of mortality by univariate analysis, but the relevance of the V-MIC and the presence of hVISA remain uncertain. A multicenter prospective randomized study by the use of standardized methods is needed to evaluate the relevance of hVISA and determine the optimal treatment of patients whose isolates have V-MICs of ≥2.0 µg/ml.

* Corresponding author. Mailing address: St. John Hospital and Medical Center, 19251 Mack Avenue, Suite 340, Grosse Pointe Woods, MI. Phone: (313) 343-3802. Fax: (313) 343-7840. E-mail: riad.khatib{at}stjohn.org

{triangledown} Published ahead of print on 15 April 2009.

Journal of Clinical Microbiology, June 2009, p. 1640-1644, Vol. 47, No. 6
0095-1137/09/$08.00+0     doi:10.1128/JCM.02135-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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