antibioticsbooks

chapter 3 Pharmacokinetics 29
because of obesity, but the drug does not distribute
well into excess adipose tissue). Thus, you may see
recommendations for antibiotic dosing based on
total or actual body weight, ideal body weight (an
estimate of the patient’s body weight without their
excess adipose tissue), or adjusted body weight
(a value between the ideal and total body weight).
Finally, it is important to note that with few
exceptions, microbiological susceptibility testing
does not account for distribution and is based on
achievable bloodstream concentrations. For example,
the microbiology lab may determine that an
organism with a minimum inhibitory concentration
(MIC) of 4 mcg/ml is considered susceptible
to a drug that achieves a concentration of 8 mg/L
in the bloodstream, but it may only achieve a concentration
of 1 mg/L in cerebrospinal fluid. Thus,
that drug would be likely to work for a bloodstream
infection caused by the organism, but would fail for
meningitis where cerebrospinal fluid concentrations
are important. Thus, distribution is important
to keep in mind when choosing antibiotics.
Metabolism/Excretion
Many antibiotics are excreted from the body,
either in the urine or feces, mostly in the same
form as they were administered. In fact, shortly
after penicillin was developed and supplies
were scarce, doctors used to collect the urine of
patients who received penicillin and recrystallize
the drug for use in other patients! When a
drug is excreted mostly unchanged, it can reach
very high concentrations in tissues in the elimination
pathway, making it potentially more effective
for infections in those systems than would