antibioticsbooks

36 PART 1 Antibiotic Therapy
Table 4–1
Examples of Antibiotic Susceptibility Breakpoints
Organism
Antibiotics
E. coli
Susceptible
(Breakpoint) Intermediate Resistant
Cefepime # 8 mcg/ml 16 mcg/ml $ 32 mcg/ml
Levofloxacin # 2 mcg/ml 4 mcg/ml $ 8 mcg/ml
Trimethoprim/ # 2/38 mcg/ml — $ 4/76 mcg/ml
sulfamethoxazole
Streptococcus
pneumoniae
Cefepime
(meningitis)
(Nonmeningeal)
# 0.5 mcg/ml
# 1 mcg/ml
1 mcg/ml
2 mcg/ml
$ 2 mcg/ml
$ 4 mcg/ml
Levofloxacin # 2 mcg/ml 4 mcg/ml $ 8 mcg/ml
Trimethoprim/
sulfamethoxazole
# 0.5/9.5 mcg/ml 1–2/19–38
mcg/ml
$ 4/76 mcg/ml
to cefepime, it does not mean that levofloxacin
is a better choice for that patient. Levofloxacin is
a concentration-dependent drug that is typically
dosed in amounts of 500–750 mg daily. Cefepime
is a time-dependent drug that is typically dosed as
1–2 g every 8 to 12 hours. The much higher concentrations
of cefepime achieved in the body (due
to higher doses) mean that organisms with a higher
MIC to cefepime are still susceptible to it. In other
words, the two numbers are not directly comparable.
In fact, if the MIC was 8 mcg/L to both drugs,
the organism would be considered resistant to levofloxacin
and susceptible to cefepime.
Finally, be aware that other methods of susceptibility
testing exist, including disk diffusion and
E-tests, but that broth dilution methods are generally
considered the gold standard.