Choosing the best anti-Helicobacter pylori therapy: effect of antimicrobial resistance. 
The development of effective therapies for the treatment of Helicobacter pylori infection has been a long and arduous process.  There is considerable confusion about which is the best.  We review approaches to understanding the results of trials evaluating potentially new therapies and those comparing two or more potentially good regimens with particular emphasis on the role of antimicrobial resistance on the outcome of therapy.  Antimicrobial resistance in vitro does not always correlate with poor results with multi-drug treatment regimes.  Although it is not known if clinical resistance defined as failure of a therapy might be a better predictive of subsequent failure, overall effectiveness is influenced most strongly by the drug with the poorest cure rates in the presence of resistant microorganisms.  Development of resistance is reduced in multiple drug therapies.  Bismuth may be an especially useful antimicrobial in this regard because it is an effective topical therapy that markedly reduces the bacterial load.  Resistance to antimicrobials can be thought of as a statistical event estimated as a proportion of the H.pylori population in the stomach (e.g., 1 in 10(8) bacteria).  Elimination of most organisms with the first dose of bismuth might decrease likelihood of survival of resistant strains that were already present.  Results of large clinical trials are needed to provide accurate estimation concerning the effectiveness of the different treatment regimes using different dosages, dosing intervals, and duration of therapy.  For interpretation and comparison, clinical trials must report the overall effectiveness as well as effectiveness of the regimen separately for those with resistant and sensitive H.pylori.