ARUP's Laboratory Test Directory
| 0090141: Phenytoin, Free & Total |
| Test Mnemonic: FDIL | |
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Methodology: Immunoassay
Performed: Sun-Sat Reported: 1-2 days |
| Specimen Required: | |
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Collect: One 5 mL plain red.
Transport: 2 mL serum at 2-8°C. (Min: 1 mL) Submit specimen in an ARUP Standard Transport Tube. Pediatric Collection/Transport: 1 mL serum at 2-8°C. Remarks: Separate serum from cells ASAP. Avoid use of separator tubes and gels. Stability: After separation from cells: Ambient: 4 days; Refrigerated: 4 days; Frozen: 1 month |
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| Reference Interval: | ||||||||||||
Therapeutic Range:
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| Interpretive Data: | |
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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.
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| CPT Code(s): | |
| 80185 Phenytoin, total; 80186 Phenytoin, free |