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.
