UW Hospital and Clinics Lab Test Directory
| Test Name: |
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Phenytoin by HPLC
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| Test Code(s): |
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LCPTN / HCLCPTN
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| CPT Code(s): |
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80185
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| Methodology: |
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Liquid Chromatography
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| Clinical Significance: |
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Anticonvulsant
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| Days Performed: |
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Mon-Fri, dayshift.
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| Turnaround Time: |
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Stat: Not available stat.
Routine: 24 hours.
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Specimen Requirements
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| Specimen: |
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Blood
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| Collection Instructions: |
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Draw specimen 18-24 hours after completion of PO loading dose, or 30 minutes prior to next dose.
With IV administration, draw specimen 2-4 hours after loading dose.
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| Collection Container: |
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Preferred: Red top
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| Collection Volume: |
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Preferred: 3 mL
Pediatrics: 1 mL
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| Sample Analyzed: |
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Serum
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| Volume Required: |
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Preferred: 1 mL
Pediatrics: 0.5 mL
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| Specimen Processing: |
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Centrifuge. Transfer cell-free serum to plastic vial. Refrigerate.
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| Specimen Transport: |
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Transport specimen to UWHC Core Laboratory (B4/220). Transport with coolant pack if coming from outreach location.
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| Unacceptable Criteria: |
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Specimens collected in red cap with yellow ring (SST) tubes are not acceptable.
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| Stability: |
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Refrigerated: 3 days
Frozen: Indeterminate
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Interpretation
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Interpretive Guidelines:
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Generally effective therapeutic concentration:
18 years and up: 10.0-20.0 mcg/mL
Toxic:
18 years and up: >25.0 mcg/mL
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| Critical Calls: |
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View
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| Additional Information: |
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Result of BELOW indicates that level is below method detection limit of 0.5 mcg/mL. Peak serum concentrations can be seen 2-4 hours after ingestion. The half-life in an adult is 8-60 hours, and 7-29 hours in a child. Phenytoin clearance is generally considered non-linear (concentration dependent) within the therapeutic range and thus, half-life will vary between individuals and with concentration. If enzyme inducers are given simultaneously, the half-life can be reduced. Phenytoin is an enzyme inducer. Valproic acid may cause an increase in free (unbound) phenytoin concentration.
This test was developed and its performance characteristics determined by the UWHC Clinical Laboratories. This test meets the standards for clinical testing. This test should not be regarded as investigational or for research use. The US Food and Drug Administration (FDA) has not approved or cleared this test; however, the FDA has determined that such clearance or approval is not necessary.
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Fosphentoin Cerebyx Dilantin
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