Detection of antibiotic resistance in vitro

Testing the susceptibility of bacteria to antimicrobial agents is fundamental to the study of resistance. The qualitative methods that were originally devised evolved separately in different countries and as the need for more precise quantitative information became clear, different solutions to the problem emerged. Consequently, the methods now recommended vary widely between countries and achieving a consensus on standardisation has proved difficult. Sensitivity testing serves two purposes to provide meaningful results to the prescriber and to monitor changes in susceptibility of microbial populations. The diagnostic purpose is adequately served by the diversity of methods used, provided they are performed and controlled efficiently. Highly standardised methods are however needed for surveillance in the national and international context and to provide meaningful comparisons to be made between individual centres. These needs are best met by the use of agreed methods and interpretative criteria in sentinel laboratories. No sensitivity test method can provide the 'correct' answer all the time, if only because the 'correct' answer in therapeutic terms is not known with any certainty. Laboratories providing a sensitivity testing service can influence prescribing--and hence the development of resistance--in a number of different ways and the quest for standardisation at all costs should not obscure these important functions.