[Considerations on the sense and nonsense of sensitivity determinations for veterinary practice]

The parameter most frequently used as a basis for antibiotic therapy is the minimum inhibitory concentration (MIC). This is a valid, standardised and comparative parameter for evaluating the effectiveness of a drug under laboratory conditions. The results of this test are considerably affected by changes in the standard laboratory conditions, which differ widely from the varying in vivo situations. The most common methods used in sensitivity testing, which are based on MIC values and pharmacokinetic data are discussed. The minimum antibiotic concentration (MAC) is much less affected by laboratory manipulations than is the MIC. The MAC is the lowest concentration having a barely measurable effect on the bacterium, whatever method is used. Although the MAC has not yet been standardised, it is a very informative test. The MAC is usually much lower than the MIC and it clearly shows that the MIC is an underestimation of antibacterial activity, particularly in animals with a reasonably functioning natural resistance. However, antimicrobial therapy should not be evaluated by microbiological and pharmacological methods alone, but also by clinical parameters, as the clinical response to antibacterial treatment may be influenced by many factors, which cannot be calculated as yet. Until he or she initiates treatment, particularly in life-threatening situations, a veterinary practitioner cannot wait for laboratory results. However, by carefully evaluating the clinical symptoms, and as a result of training and experience, he or she should be able to determine the causative organism in the majority of cases. Making and evaluating Gram-stained smears may be extremely helpful. The species of the suspected causative organism will suggest the antibiotic treatment of choice. Prospective sensitivity testing and resistance monitoring of pathogenic bacterial species, which may readily become resistant, at regular intervals is likely to be more effective, than culturing and testing alone in the event of failure of therapy.