Mixed Order Kinetic

Is seen when the elimination process is saturated.
The rate of elimination is at its maximum and it does not increase with increasing plasma concentration.

Is seen when the elimination process is saturated over the therapeutic concentration of the drug.
The elimination kinetic switches between first-order and zero-order kinetic.

At lower concentrations

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the elimination process is not saturated yet

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the drug follows first-order kinetic

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with increasing concentration → rate of elimination increases

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as the concentration of the drug increases further

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elimination process gets saturated

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kinetic switches from first order to zero order

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rate of elimination reaches the maximum and it does not increase with plasma concentration

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as the concentration of drug falls

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kinetic switches from zero order to first order

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thus kinetic switches between first and zero order depending on the concentration of the drug

Its also called:
- Saturable kinetic
- Capacity limited kinetic
- Nonlinear kinetic
- Michaelis Menten elimination kinetic
Examples of drugs showing mixed order kinetic:
- Phenytoin
- Digoxin
- Warfarin

It produces an unpredictable pharmacokinetic state. e.g. plasma half-life changes with dose.
May produce toxicity.

Once the elimination process is saturated

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a small increase in the dose

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result in a marked increase in concentration

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may produce toxicity

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therapy with such drugs should be carefully monitored.

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