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15. Empiric Therapy

His microbiology review helped Jeff understand when antimicrobial therapy is warranted. "But what do you do if there is no culture or antimicrobial susceptibility information?" he asked.

Dr. Mitchell explained that veterinarians routinely treat empirically if it is impractical to get diagnostic samples or if treatment needs to begin before culture and susceptibility results are available.

Empiric therapy

Drug selection based solely on experience and relevant clinical observational information including knowledge of current resistance patterns in suspected pathogens.

Empiric therapy is commonly used in veterinary and human medicine. For example, when a dog presents with symptoms of a urinary tract infection, the veterinarian may initiate empiric therapy (i.e. amoxicillin with clavulanate) based on prior experience with similar infections. If a urine sample is collected and submitted to a clinical laboratory, empiric treatment may be initiated before culture and sensitivity results are known. Such laboratory results are helpful in establishing antibiograms to help veterinarians decide which antimicrobial therapy should be used for future empiric therapy (see diagram below).  In human medicine, local antibiotic resistance patterns are often published and updated regularly by health departments or hospitals.


A chart listing pathogens and their susceptibility to an array of antimicrobials – based on laboratory susceptibility testing at one clinical laboratory.

(Number of Isolates Tested)
Escherichia coli
Staphylococcus aureus
Pasteurella multocida

Dr. Mitchell showed Jeff an example of an antibiogram based on clinical isolates tested at his veterinary school's diagnostic laboratory.

"Looking at this antibiogram, I can see that enrofloxacin would be the best choice for a probable P. multocida infection, since 98% are susceptible and this is better than any other antibiotic,” Jeff remarked.

"It’s not that simple," Dr. Mitchell replied. "Yes – P. multocida are more likely to be susceptible to enrofloxacin than the other antibiotics, but that class of drug is important for human medicine and should be saved for treatment failures. With ampicillin also being highly effective and if the infection caused by P. multocida was not life threatening, it would be better to use ampicillin as a first-line drug and save newer generation drugs, like enrofloxacin, for more severe or highly resistant infections."


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