The antimicrobials that NARMS uses are selected based on their importance in human and veterinary medicine and for their utility as epidemiological markers for the movement of resistant organisms and genes between environments. NARMS tests for susceptibility to 14 antimicrobials in Salmonella and E. coli, 9 in Campylobacter, and 14 in Enterococcus.

In its Guidance for Industry entitled ‘Evaluating the Safety of Antimicrobial New Animal Drugs with Regard to Their Microbiological Effects on Bacteria of Human Health Concern’ (GFI #152), FDA provides recommendations on how to rank antimicrobials with their relative importance in human medicine. Antimicrobial agents are categorized based on the following criteria 1) used to treat enteric pathogens that cause foodborne disease, 2) sole therapy or one of few alternative to treat serious human disease or drug is essential component among many antimicrobials in treatment of human disease, 3) used to treat enteric pathogens in non-foodborne disease, 4) no cross resistance within drug class and absence of linked resistance with other drug classes, 5) difficulty in transmitting resistance elements within or across genera and species of organisms. Based on these relevant factors, drugs are ranked as C- Critically Important, H- Highly Important, or I- Important. Following these rankings, resistance patterns are evaluated and reported differently depending on the consequences to public health of individual drug classes.