Pearl: When reading the antibiotic sensitivity chart for your patient’s current (or former) infection, you cannot use the MIC numerical values to compare antibiotics. Just stick to sensitive/resistant.

Question: When you look up the antibiotic sensitivities for a patient’s cultured organism the table always lists MIC (minimum inhibitory concentration) ratio values (1:2, 1:8, 1:5) to the left of the interpretation for each antibiotic (sensitive vs resistant). Among sensitive antibiotics, can you use the MIC numbers to pick the optimal antibiotic?

Answer: The MIC cutoff for resistance is specific to each pathogen-antibiotic pairing. For this reason, the numbers cannot be compared to each other. From our perspective, sensitivity/resistance is binary. The numbers are just there to confuse us.

Special thanks to the friendly director of the MSH Microbiology Lab who answered my questions this afternoon.