According to our laboratory policy, we do not repeat antimicrobial susceptibility tests if the same species of bacteria is recovered from the same anatomic site from the same patient within four days.  However, I have been unable to find a recent literature citation to support this practice. Do you know of one?

Answer:

NCCLS M100-S12 states in several places (e.g., Tables 2A, 2C, and 2B) that organisms can become resistant to an antimicrobial agent within 3 to 4 days after initiation of therapy. Thus, repeating the susceptibility tests for isolates recovered 4 or more days after the initial result may be warranted.  In particular, this pertains to Enterobacter, Citrobacter, and Serratia species tested against third-generation cephalosporins; staphylococci tested with fluoroquinolones; and Pseudomonas aeruginosa tested with beta-lactam agents.  General practice is to repeat the susceptibility testing on all significant organisms every four or more days.