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17. Using MIC values

"Ok, but how do I use the MIC?” Jeff asked.



A MIC is valuable in calculating the proper dose of the antibiotic. When an antibiotic is developed, the common pathogen(s) of interest are used to establish the dose given the antibiotic's properties. However the “label dose” should not always be relied on because certain bacterial strains have developed different levels of resistance over time. "Let's go through an example based on Fernando's case.” Dr. Mitchell remarked.

Example antibiotic calculation:

Dose= (Cmax * Vd) / F

Cmax = highest concentration in plasma, calculated as
MIC X 2(dose interval/half-life)

Vd = volume distribution. Comparison of amount of drug in body versus the plasma concentration.

F = Systemic availability. Fraction of a drug that reaches the blood unchanged. The route of administration being given typically determines this value (e.g., IV=1, IM=0.7)

*All can be found on the antibiotic’s insert or pharmacology reference.

Example antibiotic dosage calculation: 

Drug to use

Pharmacokinetic parameters Vd: 1.4 L/kg
t1/2: 12 hours (IV)
Target MIC: 64.0 µg/ml for Samlonella Newport
Dose interval of 24 hours

Dose interval / t1/2 = 24/12 = 2
**(Drug will go through 2 half-lives at this regimen)

Cmax = MIC * 2dose interval/drug half-life

Cmax = 64 * 22 = 256 µg/ml

**(Plasma concentration required to maintain a plasma level above 64mg/ml for 24 hours)

Calculate dose

Dose = Cmax * Vd

        = 256 µg/ml * 1.4 L/kg
        = 358.4 mg/kg

**(Dose required for a 24 hour period, without taking into affect the drug’s systemic availability.)

Systemic availability

IV administration: F = 1.0
Dose = Cmax * Vd ÷ F

        = 358.4 mg/kg ÷ 1.0

Final dose = 358.4 mg/kg

**(Dose required for Tetracycline to be administered IV for a 24 hour period.)

In this example, the calculated dose for oxytetracycline would be 358.4mg/kg for this particular strain of Salmonella, even though the label says 5 – 7.5 mg/kg. A dose of this magnitude for a 400 kg horse would be 317 mls of LA-200, which is not a practical or advisable dose. Therefore, another drug should be used.



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