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Overview :
The therapeutic ratio for an antibiotic is the serum level of
the bactericidal antibiotic divided by the minimal inhibitory concentration
(MIC) of the organism being treated.
antibiotic serum level-to-MIC ratio =
= (serum level of antibiotic after a dose) / (MIC)
For bloodstream or soft tissue infections, a therapeutic
ratio of 4 :1 to 8 :1 is usually sufficient, whereas for pulmonary infection, a
ratio of 16: 1 may be required to provide adequate tissue levels, since there is
poor penetration of antibiotics into sputum.
For penicillin, this ratio is typically high and easy to
achieve. For example, the average MIC for penicillin against Streptococcus group
A or S. pneumoniae is 0.015-0.020 µg/mL. With low‑dose penicillin,
therapeutic ratio =
= approximately (6.0 µg/mL) / (0.02 µg/mL)
= 300:1 therapeutic ratio.
For aminoglycosides, it is often a problem to achieve
adequate therapeutic ratios. If K. pneumoniae's MIC for gentamicin is
approximately 1.56 µg/mL
The therapeutic ratio with average serum levels =
= (6 µg/mL) / (1.56 µg/mL)
= about a fourfold ratio
This therapeutic ratio of 4:1 is adequate, in theory, for
bloodstream infections. However, this peak is maintained for less than 1‑2
hours, and suboptimal dosing is not uncommon with aminoglycosides. Even lower
concentrations of the antibiotic are found at tissue levels.
A related problem with the use of aminoglycosides is the
narrow margin between effective and toxic concentrations.
• see [Therapeutic Index]
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