When doing a susceptibility test with sulfonamides, there are two concentric zones around the disk. How should the zone be measured?

In the context of susceptibility testing, particularly with sulfonamides, the presence of two concentric zones around the disk indicates the formation of an inner zone of inhibition and an outer zone, which is typically a result of the diffusion of the antibiotic and the bacterial response. When measuring these zones, it is standard practice to measure the outer zone because this zone represents the broader area of complete inhibition of bacterial growth.

The outer zone provides a more accurate reflection of the antibiotic's effectiveness, as it encompasses the range in which the bacteria can no longer survive or reproduce due to the presence of the sulfonamide. In contrast, measuring only the inner zone could lead to an underestimation of how effective the antibiotic is against the tested strain, as the inner zone may not account for variations in bacterial susceptibility across a broader area.

Thus, measuring the diameter of the outer zone ensures that the results are more representative of the drug's impact on the organism being tested, facilitating better clinical decision-making related to treatment options.