antibioticsbooks

Appendix
2
Spectrum of
Activity
A Note on Spectrum of Activity
Spectrum of activity is usually thought of as “does
this drug cover this bug?” It’s important to realize
that this is a substantial oversimplification.
A more accurate statement would be, “What is the
probability, in this patient, that the pathogen I am
concerned about is susceptible in vitro to this antibiotic?”
The key components are “probability”
and “in this patient.” Some antibiotics are always
active against some organisms (e.g., penicillin and
Streptococcus pyogenes) and a fair number of
antibiotics are never active against some organisms
at safe concentrations (e.g., vancomycin and
Escherichia coli). But it is more common that
there is some degree of variability in susceptibility
across different isolates of the organism, and that
there is variability in this variability! For example,
Table A–1 describes the susceptibility of E. coli to
ciprofloxacin at three hospitals—Major Medical
Center, Trauma Hospital, and the Regional
Children’s Hospital—all located in the same city
(this example is based on actual data, but the
names have been changed to protect the “innocent”
institutions).