Ivy Tech Microbiology Lab Test 2 Practice

Interpreting disk diffusion results relies on breakpoints that define the cutoff zone diameters for each antibiotic-organism combination. After incubation, you measure the zone of inhibition and compare it to these breakpoints. If the zone meets or exceeds the susceptible breakpoint, the organism is considered susceptible; if it falls into the intermediate range, it’s intermediate; if it’s below the resistant breakpoint, it’s resistant. Breakpoints are established by standards bodies and depend on the specific antibiotic, organism, and testing method, so they’re essential criteria, not optional or irrelevant. While inoculum size can affect zone size, breakpoints themselves are the interpretive thresholds that translate the measured zone into a clinical category.