Evaluation of Methods in Detecting Vancomycin Mic Among Mrsa Isolates and the Changes in Accuracy Related to Different Mic Values

Abstract

INTRODUCTION: Methicillin-Resistant Staphylococcus aureus (MRSA) presenting reduced susceptibility to vancomycin has been associated to therapeutic failure. Some methods used by clinical laboratories may not be sufficiently accurate to detect this phenotype, compromising results and the outcome of the patient. OBJECTIVES: To evaluate the performance of methods in the detection of vancomycin MIC values among clinical isolates of MRSA. MATERIAL AND METHODS: The Vancomycin Minimal Inhibitory Concentration was determined for 75 MRSA isolates from inpatients of Mãe de Deus Hospital, Porto Alegre, Brazil. The broth microdilution (BM) was used as the gold-standard technique, as well as the following methods: E-test® strips (BioMérieux), M.I.C.E® strips (Oxoid), PROBAC® commercial panel and the automated system MicroScan® (Siemens). Besides, the agar screening test was carried out with 3 µg/mL of vancomycin. RESULTS: All isolates presented MIC ≤ 2 µg/mL for BM. E-test® had higher concordance (40%) in terms of global agreement with the gold standard, and there was not statistical difference among E-test® and broth microdilution results. PROBAC® panels presented MICs, in general, lower than the gold-standard panels (58.66% major errors), while M.I.C.E.® MICs were higher (67.99% minor errors). CONCLUSIONS: For the population of MRSA in question, E-test® presented the best performance, although with a heterogeneous accuracy, depending on MIC values.

Vancomycin MIC Does Not Predict 90-Day Mortality, Readmission, or Recurrence in a Prospective Cohort of Adults with Staphylococcus aureus Bacteremia

“…The use of a MicroScan system to determine MIC potentially underestimates or overestimates values depending on the standard to which it is compared (19,20,32,(60)(61)(62)(63)(64)(72)(73)(74), which leads to nondifferential misclassification of the exposure. Some data have suggested that assessing the risk for mortality may be predicted more accurately by nonautomated systems for determining MIC than by automated systems such as MicroScan (32).…”

“…This is particularly important for individuals who have complicated disease associated with higher rates of mortality or recurrence but who also require longer courses of antibiotic therapy. Also, although potentially more scientifically accurate, several of these studies assessed vancomycin MICs with assays that are rarely used in routine clinical practice, which limits the generalizability of findings from such studies (15,20,(57)(58)(59)(60)(61)(62)(63)(64). Finally, understanding how treatment failure affects outcomes other than mortality, including readmission or SAB recurrence is of interest and should also be considered.…”

Vancomycin susceptibility trends of methicillin-resistant Staphylococcus aureus isolated from burn wounds: a time for action

“…This is a highly sensitive and specific method for hVISA detection, that can be easily implemented in the clinical microbiology laboratory [8,21]. It has been found that this method has higher agreement with broth microdilution, presenting the most homogeneous performance of different MIC values [22]. Reiderer and colleagues suggested that the high sensitivity of Etest GRD strips may make it a useful screening procedure for detection of VISA or hVISA, but confirmatory testing would be required [23].…”

Evaluation of vancomycin MIC creep in methicillin-resistant Staphylococcus aureus infections—a systematic review and meta-analysis

“…In this context, and as a result of the reported vancomycin therapy failure in patients with S. aureus infections with a MIC !4 mg/L, the CLSI reduced vancomycin breakpoints from 4 mg/L to 2 mg/L, for susceptible S. aureus, and from !32 mg/L to !16 mg/L for resistant S. aureus. These changes aimed to increase the sensitivity of the detection of non-susceptible isolates [3,7]. The apparent increase in vancomycin MIC among MRSA, observed in recent years, can represent the first step towards the appearance of fully resistant isolates.…”