A study made by Li J. et al. (2017) investigated how some factors influence antimicrobials' efficacy. Some of these factors include the following:

• Antibiotic Concentration: The concentration of antibiotics in discs or wells should be consistent with established standards. Deviations can lead to incorrect results.
• Inoculum size: The quantity of bacteria used in AST can influence the size of the inhibition zones and affect susceptibility interpretations.
• Resistance Mechanisms: Bacteria may possess various resistance mechanisms (e.g., efflux pumps beta-lactamase production) that are not always detected through standard AST methods. Testing Methodology: Variations in testing methods (e.g., disc diffusion, broth microdilution, E-test) can yield different results.

Clinical laboratories follow standardized protocols and quality control measures to minimize the impact of these factors and ensure accurate antimicrobial susceptibility testing results. Regular proficiency testing and adherence to guidelines from organizations like the Clinical and Laboratory Standards Institute (CLSI) help maintain the reliability of AST.

Reference: Li, J., Xie, S., Ahmed, S., Wang, F., Gu, Y., Zhang, C., Chai, X., Wu, Y., Cai, J., & Cheng, G. (2017). Antimicrobial Activity and Resistance: Influencing Factors. Frontiers in pharmacology, 8, 364. https://doi.org/10.3389/fphar.2017.00364

For antimicrobial susceptibility testing alot of factors can affect your result especially when you are using the diffusion method. These factors are the following but not limited to just these;

PH of the medium

Temperature

Inoculum size

Concentration