7
mm
314.16
mm²
28.27
mm²
285.88
mm²
7
mm
314.16
mm²
28.27
mm²
285.88
mm²
The Zone of Inhibition Calculator computes the dimensions of the clear zone around an antibiotic disc in a Kirby-Bauer disc diffusion susceptibility test. The zone of inhibition is the area around the disc where bacterial growth is prevented by the antibiotic diffusing from the disc into the agar. Larger zones generally indicate greater bacterial susceptibility to the antibiotic.
This test is the most widely used method for antibiotic susceptibility testing in clinical microbiology laboratories worldwide. Zone diameters are compared to standard breakpoints to classify organisms as susceptible, intermediate, or resistant.
The calculator provides several measurements:
The measured diameter is the total distance across the clear zone including the disc. Zone interpretation depends on the specific antibiotic and organism combination, as defined by CLSI or EUCAST breakpoint tables.
Inputs
Results
A 22 mm zone for ampicillin against E. coli is above the susceptible breakpoint (>=17 mm per CLSI), indicating the organism is susceptible.
Inputs
Results
A 14 mm zone may be in the intermediate range depending on the antibiotic-organism combination. Consult CLSI breakpoint tables for interpretation.
Zone diameters are compared to published breakpoint tables (CLSI or EUCAST) that define susceptible (S), intermediate (I), and resistant (R) categories for each antibiotic-organism combination. For example, for ciprofloxacin against Enterobacteriaceae: >=21 mm is S, 16-20 mm is I, and <=15 mm is R. Always use current guidelines for your specific organism and antibiotic.
Zone size depends on antibiotic diffusion rate (molecular size, solubility), agar depth and composition, inoculum density, incubation time and temperature, and the minimum inhibitory concentration of the organism. Standardized protocols (Mueller-Hinton agar, 0.5 McFarland inoculum, 35°C, 16-18 hours) minimize variability.
Not necessarily. Zone size reflects both antibiotic potency and diffusion characteristics. Different antibiotics have different breakpoints; a 15 mm zone might be susceptible for one antibiotic but resistant for another. Zone size cannot be directly compared between different antibiotics. Always compare to the appropriate breakpoint table.
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