The normal values for phenytoin are 10 – 20mcg/ml, and this range is unisex for all age groups.
Test results may vary depending on the person’s age, gender, health history, the method used for the testing and co-administrated medications.
The therapeutic ranges for most adults have been established at 10.0-20.0 mcg/mL for total phenytoin and 1.0-2.0 mcg/mL for free phenytoin.
This reference range for unbound phenytoin was established based on an assumed 10% unbound drug fraction.
However, the free fraction of phenytoin can vary from 3% to 37% among patients.
In general, when phenytoin results are in the therapeutic range, most patients will respond to the drug without symptoms of toxicity.
Response and side effects will be individual, however. Some people will experience seizures at the low end of the therapeutic range and some people will experience excessive side effects at the upper end.
People should work closely with their healthcare practitioner to find the dosage and concentration that works the best for them. Dosage determinations and adjustments must be evaluated on a case-by-case basis.