RISKS RELATED TO LACK OF STANDARDIZATION OF TESTS TO DETECT IN-VITRO METRONIDAZOLE RESISTANCE IN HELICOBACTER-PYLORI

The correlation between metronidazole MIC readings obtained in vitro o n two different media for two different inoculum dilutions was studied after two, three, and five days of incubation using 20 clinical isola tes of Helicobacter pylori. The PDM epsilometer (E test) was used. Aft er two days of incubation, log MIC values on chocolate PDM and Vest-fo ld charcoal medium, a new charcoal medium, showed good correlation. Ch arcoal media can thus be used for metronidazole sensitivity tests. In terms of double-dilution gradients, prolongation of incubation time fr om two to three days and change of inoculum dilution from 0.5 to 4 McF arland had little impact when studied separately. However, the combine d effect of the two variables was marked. The matching of a sparse ino culum incubated for two days against a heavy inoculum incubated for th ree days resulted in readings which, for 30% of the pairs, differed by two double-dilution gradients or more. On the other hand, the separat e influence of both variables was found to affect the correlation coef ficients significantly. Five days of incubation was associated with a poor correlation when the variables were studied both separately and c ombined. Thus, the real influence of separate variables on MIC reading s was not exposed in a scale of double-dilution gradients because the intervals were too large. For this reason, scales without thresholds a re recommended.