Test Details & Preparation

• Phenytoin test is done to determine the level of the drug phenytoin in your blood in order to maintain a therapeutic level and to detect potential for toxicity.


• Phenytoin test is performed at regular intervals to monitor as needed to detect low or toxic concentrations.


• A blood sample drawn from a vein in your arm for a Phenytoin test.


• No test preparation is required for a Phenytoin test.


• The therapeutic ranges for adults taking phenytoin have been established at 10.0-20.0 mcg/mL for total phenytoin (bound plus unbound) and 1.0-2.0 mcg/mL for free phenytoin (unbound only). The range for percent free phenytoin is 8-14%.


• Within these ranges, most people will respond to the drug without symptoms of toxicity. Response and side effects will be individual, however.


• In general, when phenytoin results are in the therapeutic range, the person is not having recurrent seizures and is not experiencing significant side effects then the dosage of phenytoin is adequate.