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Overview :
Ellner and Neu described the inhibitory quotient, which is
the ratio of an antibiotic concentration in a body fluid to the minimum
inhibitory concentration (MIC). This is the multiple of the MIC that should be
achieved with an antibiotic dosage regimen. The authors are from Columbia
University College of Physicians and Surgeons in New York City.
NOTE: The inhibitory quotient for serum is the same as the
"antibiotic serum level-to-MIC ratio" described in the previous section.
Relevant body fluids:
(1) serum
(2) urine
(3) bile
(4) cerebrospinal fluid (CSF)
Parameters:
(1)
measured or estimated peak antibiotic concentration in a body fluid in µg/mL
(2) minimum
inhibitory concentration (MIC) of the antibiotic for the bacterial isolate in
µg/mL
inhibitory quotient =
= (peak antibiotic concentration in the body fluid in µg/mL)
/ (MIC in µg/mL)
where:
• If an
antibiotic is highly protein bound then the free drug concentration should be
used (page 1577).
• Ideally a
level above the MIC should be achieved for a 2-hour period (page 1577).
• Measuring
the antibiotic concentration in the fluid is indicated if the achieved levels
are variable due to concurrent conditions (urine with renal failure, CSF with
meningeal inflammation, etc.).
Interpretation:
• The
higher the inhibitory quotient the more likely the antibiotic therapy will be
effective.
• An
inhibitory quotient < 1 is unlikely to be effective.
• In
general an antibiotic should be considered for use if its inhibitory quotient is
at least 4-8 (page 1577, column 1). For infections like endocarditis an
inhibitory quotient of 8 or greater may be appropriate.
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