Inadequate mixing in microdilution wells during preparation of antimicrobial dilutions for susceptibility testing

An equation representing a twofold series of antibiotic dilutions was derived. The theoretical results were then validated with experimental observations of piperacillin against Pseudomonas aeruginosa. Insufficient mixing leads to an erroneously low MIC. It is concluded that maintaining antibiotic concentration equilibrium during microdilution is required for accurate microdilution susceptibility tests.