Kirby-Bauer (KB) tests are performed under standardized conditions and standard-sized zones of inhibition have been established for each antibiotic. If the observed zone of inhibition (ZoI) is greater than or equal to the size of the standard zone, the microorganism is considered to be sensitive to the antibiotic. Conversely, if the observed zone of inhibition is smaller than the standard size, the microorganism is considered to be resistant. How are ZoI standards for antimicrobial disc susceptibility tests set?