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Fosphenytoin is a highly water soluble phosphate ester of phenytoin that can injected IV or IM with very few side effects. It is believed to represent a significant advance in the treatment of acute seizure disorders. Fosphenytoin is converted to phenytoin by both blood and tissue phosphatases. Therapeutic phenytoin concentrations can be achieved 10 minutes after IV and 30 minutes after IM injections.

Therapeutic drug monitoring is simplified if samples are drawn after all the fosphenytoin has been converted to phenytoin. Blood samples should be drawn two hours after completion of an IV infusion and four hours after an IM injection.

Phenytoin is the appropriate test for monitoring fosphenytoin. Therapeutic range is the same as phenytoin, 10 - 20 ug/mL.

Specimen requirement is one plain red top tube of blood.

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