The zone of inhibition is a circular area around a spot where an antimicrobial agent has been applied, where bacteria colonies do not grow. It is a qualitative method used to measure the ability of an antimicrobial agent to inhibit the growth of microorganisms. The zone of inhibition test is a fast and inexpensive way to determine antibacterial activity. The size of the zone of inhibition is usually related to the level of antimicrobial activity present in the sample or product – a larger zone of inhibition usually means that the antimicrobial is more potent. However, a large zone of inhibition does not necessarily indicate a well-protected product, as it might be a sign of a leaching antimicrobial agent, whereby the active ingredient leaves a treated product and disperses into the surrounding environment. The zone of inhibition method is qualitative and can only be used to classify the microbial strains as being susceptible, intermediate, or resistant.