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16. Pharmacology

Dr. Mitchell was pleased with Jeff's knowledge of antibiotics. He gave him a copy of Fernando's laboratory report and asked Jeff if he had any questions.

MIC (minimum inhibitory concentration)

The lowest concentration of an antimicrobial agent that prevents visible growth of a microorganism on agar or in broth. (NCCLS, M31-A2)


Zones of inhibition or MICs at which an organism is considered to be susceptible, intermediate, or resistant based on obtainable serum concentrations of the drug and clinical trials. (Antimicrobial Therapy in Veterinary Medicine, 3rd edition)






As Jeff reviewed Fernando’s laboratory report, he asked Dr. Mitchell, "I understand "S", "I", and "R" stand for “sensitive”, “intermediate” and “resistant”, but could you remind me what MIC means?"

Dr. Mitchell explained that MIC stands for "Minimum Inhibitory Concentration" and is a valuable tool for physicians and veterinarians to use in determining a therapeutic dose. The "S", "I", and "R" commonly seen in laboratory reports only helps the clinician decide, on a qualitative basis, which antimicrobial agent(s) to consider using.  The MIC allows one to calculate a more accurate dose.

Relying solely on the "S", "I", and "R" column can be misleading because an antimicrobial agent may be close to the breakpoint. If an improper dose is given, the clinician runs the risk of treatment failure and/or resistance developing. So, unless you know the breakpoints, it is better to use the MIC in calculating a therapeutic dose.

To complicate matters even more, in veterinary medicine most of the breakpoints used are based on human clinical trials. Few of the established breakpoints in veterinary medicine have been officially validated.

Validated Breakpoints in Veterinary Medicine

Spectinomycin: Bovine respiratory disease
Penicillin/Novobiocin: Bovine mastitis

Bovine respiratory disease

Porcine respiratory disease


Bovine respiratory disease

Canine & Feline dermal, upper respiratory, and urinary tract infections

Chickens & Turkeys for Escherichia coli and Pasteurella multocida

Canine & Feline dermal and urinary tract infections
Clindamycin: Canine skin and soft tissue infections
Pirlimycin: Bovine mastitis

Bovine respiratory disease

Porcine respiratory disease

Florfenicol: Bovine respiratory disease
Tiamulin: Porcine respiratory disease




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