Reference Interval:
Therapeutic Range:
| Test Number
| Components
| Therapeutic Range
| | | Total Phenytoin
| Therapeutic: 10.0-20.0 µg/mL
Toxic: Greater than 30 µg/mL
| | | Free Phenytoin
| Therapeutic: 1.0-2.0 µg/mL
Toxic: Greater than 3.5 µg/mL
| | | Percent Free Phenytoin
| 8.0-14.0%
|
|
| Interpretive Data: |
Free phenytoin may be important to monitor in patients with altered or unpredictable protein binding capacity because phenytoin is highly bound (greater than 90%) at therapeutic concentrations. Phenytoin is also subject to drug-drug interactions due to displacement of protein binding and extensive metabolism. Cross-reactivity with metabolites may account for differences in phenytoin concentrations among analytical methods. Calculating percent free attempts to minimize differences in assay cross-reactivity and may be useful in dose optimization.
|
| CPT Code(s): |
80185 Phenytoin, total; 80186 Phenytoin, free
|
Cross References: |
Dilantin, Free (Phenytoin, Free and Total), Diphenylan (Phenytoin, Free and Total), Diphenylhydantoin (Phenytoin, Free and Total), Fosphenytoin (Phenytoin, Free and Total), Free Dilantin (Phenytoin, Free and Total), Free Phenytoin (Phenytoin, Free and Total), Phenytek (Phenytoin, Free and Total), Protein Free Phenytoin (Phenytoin, Free and Total) |