Performance of the Oxoid M.I.C.Evaluator Strips compared with the Etest assay and BSAC agar dilution

The Oxoid M.I.C.Evaluator (M.I.C.E; Thermo Fisher Scientific) comprises an antibiotic gradient on a plastic support. We compared its performance with Etest--a similar product--using BSAC agar dilution as a reference.

Parallel MIC tests were performed by M.I.C.Evaluator, Etest and agar dilution on Iso-Sensitest agar. In total, 9354 organism/strip combinations were tested by each method, using 1017 bacteria representing clinically important fastidious and non-fastidious species.

Essential agreement of strip MIC values (+/-1 doubling dilution) with the agar dilution reference, with off-scale results excluded, was 89.9% for M.I.C.Evaluator versus 89.5% for Etest (P > 0.05). These proportions were similar, at 89.5% and 89.3% (P > 0.05), respectively, if off-scale values were counted as agreeing if they could agree (e.g. a strip MIC >32 mg/L and an agar dilution MIC of 128 mg/L). For both strips, agreement with agar dilution was best for non-fastidious genera, Moraxella, Listeria, Pasteurella and Campylobacter spp. and weaker for streptococci, anaerobes, Neisseria spp. and, especially, Haemophilus influenzae. Many 'disagreements', especially for H. influenzae, concerned organisms unequivocally resistant by all methods (e.g. ampicillin MIC 256 mg/L by agar dilution, 16 or 32 mg/L by both strips); nevertheless both strips underestimated imipenem MICs for Proteus. There was no difference between the two strip types in the proportion of agreements with agar dilution (P > 0.05); nevertheless their results agreed better with each other than with agar dilution (P < 0.01).

The M.I.C.Evaluator performed almost identically to the Etest, giving good agreement with BSAC agar dilution.