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Calibration of fusidic acid disk diffusion susceptibility testing of Staphylococcus aureus

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Olsson-Liljequist B, Kõljalg S, Karlsson I, Kronvall G. Calibration of fusidic acid disk diffusion susceptibility testing of Staphylococcus aureus. APMIS 2002;110:690–6.

Single strain regression analysis, SRA, was used to calibrate disk diffusion fusidic acid susceptibility testing of Staphylococcus aureus in two laboratories using different standard methods but the same interpretative MIC limits. SRA equation constants were calculated using five different fusidic acid disk contents (1.5, 5, 15, 50, 150 μg). These disks were tested on five separate occasions against quality control strain S. aureus ATCC 29213. The National Committee for Clinical Laboratory Standards (NCCLS) method was employed in Tartu, Estonia (TE) and the Swedish Reference Group for Antibiotics (SRGA) method in Sweden at the Karolinska Hospital (KS). SRA constants obtained were used for calculating zone breakpoints corresponding to MIC breakpoints recommended by the SRGA (S ≤0.5 mg/L, R ≥1 mg/L). Zone diameter histograms from KS, performed with a 50 μg disk, and from TE, using a 10 μg disk, showed a clustering of wild type strains around 41 mm and 30 mm, respectively, reflecting differences in methodology. Zone breakpoints calculated from the equations were validated by comparison with the histograms. Breakpoints were also calculated for a suggested lower disk content in Sweden, 10 μg, and validated in tests of clinical isolates and by histogram analysis.

Keywords: Fusidic acid; antibiotic susceptibility test; disc diffusion test; disc test calibration; zone breakpoints

Document Type: Research Article


Affiliations: 1: Swedish Institute for Infectious Disease Control, Solna, Sweden, 2: Institute of Microbiology, University of Tartu, Tartu, Estonia, and 3: Clinical Microbiology – MTC, Karolinska Institute, Karolinska Hospital L2:02, Stockholm, Sweden

Publication date: 2002-10-01

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