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
|