[Rapid ATB. A new system of antibiotic testing in 4 hours. Description and parameters of variation]

Rapid ATB is a new automated system which performs antibiotic sensitivity testing of Enterobacteriaceae in four hours. It is composed of strips containing dehydrated antibiotics and a medium which ensures rapid bacterial growth, and of the equipment required for standardization of the inoculum, reading of results and interpretation. Strains are classified as susceptible or resistant according to the value of the growth index (IC) which is the ratio of growth in the presence of the antibiotic to growth of a control inoculum. Use of this system requires standardization. The main factors to control are state of the culture used to prepare the inoculum, standardization of the inoculum, incubation temperature (35 to 37 degrees C), and duration of incubation (4 to 5 hours). Variations of these factors usually induce significantly delayed growth. Under well standardized conditions, results are highly reproducible, as proved by testing ten strains ten times each by three investigators.