Abstract
BackgroundsTo determine the true incidence of hGISA/GISA and its consequent clinical impact, methods must be defined that will reliably and reproducibly discriminate these resistant phenotypes from vancomycin susceptible S. aureus (VSSA).MethodsThis study assessed and compared the ability of eight Dutch laboratories under blinded conditions to discriminate VSSA from hGISA/GISA phenotypes and the intra- and inter-laboratory reproducibility of agar screening plates and the Etest method. A total of 25 blinded and unique strains (10 VSSA, 9 hGISA and 6 GISA) were categorized by the PAP-AUC method and PFGE typed to eliminate clonal duplication. All strains were deliberately added in quadruplets to evaluate intra-laboratory variability and reproducibility of the methods. Strains were tested using three agar screening methods, Brain Heart Infusion agar (BHI) + 6 μg/ml vancomycin, Mueller Hinton agar (MH) + 5 μg/ml vancomycin and MH + 5 μg/ml teicoplanin) and the Etest macromethod using a 2 McFarland inoculum.Results and DiscussionThe ability to detect the hGISA/GISA phenotypes varied significantly between methods and phenotypes. BHI vancomycin and MH vancomycin agar screens lacked the ability to detect hGISA. The MH teicoplanin agar screen was more sensitive but still inferior to Etest that had a sensitivity of 98.5% and 99.5%, for hGISA and GISA, respectively. Intra- and inter-laboratory reproducibility varied between methods with poorest performance seen with BHI vancomycin.ConclusionThis is the first multi-center blinded study to be undertaken evaluating various methods to detect GISA and hGISA. These data showed that the ability of clinical laboratories to detect GISA and hGISA varied considerably, and that screening plates with vancomycin have a poor performance in detecting hGISA.
Highlights
Since the advent of the first glycopeptide intermediately susceptible S. aureus (GISA) and its heterogeneous variant in 1997, debate still ensues as to their clinical significance [1,2,3,4,5,6,7,8,9]
We describe the first multi-center blinded study comparing the reliability of four screening methods in an eight-center setting; namely, to assess and compare the inter-laboratory reproducibility of three "agar screening plates" and the Etest "macromethod" in discriminating vancomycin susceptible S. aureus (VSSA) from hGISA/GISA phenotypes
Instability of h-GISA phenotype All GISA and hGISA strains that consistently gave a VSSA phenotype yet were originally categorized as a GISA/ hGISA as judged by PAP-AUC, were re-sent to the central laboratory to re-confirm their original PAP
Summary
Since the advent of the first glycopeptide intermediately susceptible S. aureus (GISA) and its heterogeneous variant (hGISA) in 1997, debate still ensues as to their clinical significance [1,2,3,4,5,6,7,8,9]. This perhaps has been compounded by the discovery of the vanA-mediated glycopeptide resistant S. aureus from the US where the glycopeptide minimum inhibitory concentrations are demonstrably higher and testing issues are present, but less problematic [10,11]. Seven years after the publication of Mu50 (GISA) and Mu3 (hGISA), very little has been resolved as to which methods are the most reliable and reproducible. It is possible that hGISA and GISA strains represent the extremes of a common phenotype conferring reduced susceptibility to glycopeptides
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