Broad- versus narrow-spectrum antibiotic use: the role of in vitro testing and its correlation with clinical efficacy.

The spectrum of activity of any antibiotic varies with time, the geographical locale and the site of isolation of the bacteria. This variability is related to the changing heterogeneity (specifically, the degree of resistance) of the bacterial population. In order to choose the most appropriate antimicrobial agent, it is necessary to know the infection site, the degree of diffusion of antibiotics at this site, and the pathogen responsible, and then to determine the sensitivity of the pathogen. If the pathogen is unknown, an assessment of the patient's clinical status should be made; generally, the weaker the patient, the broader the spectrum of antibiotics that should be used. If the pathogen and its in vitro susceptibility pattern are known, narrow-spectrum antibiotics are preferable. The carefully standardized conditions of in vitro testing do not correlate with the in vivo situation, in which the effectiveness of antibiotics is altered by diffusion and immune response. The in vitro tests available do not always correlate with the clinical situation; some bacteria showing resistance in the laboratory are actually susceptible in practice in some infection sites, and vice versa. In vitro tests are also affected by technical limitations. An in vitro test can aid the assessment of efficacy but cannot guarantee it.