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Journal of Clinical Microbiology, May 1999, p. 1628-1631, Vol. 37, No. 5
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Assessment of Metronidazole Susceptibility in Helicobacter pylori: Statistical Validation and Error Rate Analysis of Breakpoints Determined by the Disk Diffusion Test

Sandra Chaves,1,2 Mário Gadanho,1,2 Rogério Tenreiro,2,* and José Cabrita1,3

Laboratório de Bacteriologia, Instituto Nacional de Saúde,1 Departamento de Biologia Vegetal e Centro de Genética e Biologia Molecular, Universidade de Lisboa,2 and Faculdade de Farmácia, Universidade de Lisboa,3 Lisbon, Portugal

Received 23 September 1998/Returned for modification 30 November 1998/Accepted 21 January 1999

Metronidazole susceptibility of 100 Helicobacter pylori strains was assessed by determining the inhibition zone diameters by disk diffusion test and the MICs by agar dilution and PDM Epsilometer test (E test). Linear regression analysis was performed, allowing the definition of significant linear relations, and revealed correlations of disk diffusion results with both E-test and agar dilution results (r2 = 0.88 and 0.81, respectively). No significant differences (P = 0.84) were found between MICs defined by E test and those defined by agar dilution, taken as a standard. Reproducibility comparison between E-test and disk diffusion tests showed that they are equivalent and with good precision. Two interpretative susceptibility schemes (with or without an intermediate class) were compared by an interpretative error rate analysis method. The susceptibility classification scheme that included the intermediate category was retained, and breakpoints were assessed for diffusion assay with 5-µg metronidazole disks. Strains with inhibition zone diameters less than 16 mm were defined as resistant (MIC > 8 µg/ml), those with zone diameters equal to or greater than 16 mm but less than 21 mm were considered intermediate (4 µg/ml < MIC <=  8 µg/ml), and those with zone diameters of 21 mm or greater were regarded as susceptible (MIC <=  4 µg/ml). Error rate analysis applied to this classification scheme showed occurrence frequencies of 1% for major errors and 7% for minor errors, when the results were compared to those obtained by agar dilution. No very major errors were detected, suggesting that disk diffusion might be a good alternative for determining the metronidazole sensitivity of H. pylori strains.

* Corresponding author. Mailing address: Departamento de Biologia Vegetal, Faculdade de Ciências da Universidade de Lisboa, Rua Ernesto Vasconcelos, Edificio C2, piso 4, Campo Grande P-1749-016 Lisbon, Portugal. Phone: 351-1-7573141. Fax: 351-1-7500048. E-mail: rpat{at}fc.ul.pt.

Journal of Clinical Microbiology, May 1999, p. 1628-1631, Vol. 37, No. 5
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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