UW Hospital and Clinics Lab Test Directory
| Test Name: |
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Phenytoin
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| Test Code(s): |
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PTN / DIL
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| CPT Code(s): |
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80185
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| Methodology: |
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Particle Enhanced Turbidometric Inhibition Immunoassay (PETINIA)
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| Clinical Significance: |
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Anticonvulsant
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| Days Performed: |
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Daily, 24 hours.
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| Turnaround Time: |
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Stat: 1 hour.
Routine: 4 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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If drug is given PO, collect specimen 18 - 24 hours after loading dose or 30 minutes prior to next dose. If drug is given IV, collect specimen within 2 - 4 hours after loading dose.
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| Collection Container: |
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Preferred: Red top
Also Acceptable:
Green top (with lithium heparin anticoagulant)
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| Collection Volume: |
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Preferred: 1 mL
Pediatrics: 0.4 mL
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| Sample Analyzed: |
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Serum or plasma
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| Volume Required: |
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Preferred: 0.5 mL
Pediatrics: 0.2 mL
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| Specimen Processing: |
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Centrifuge. Transfer cell-free serum or plasma 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) or green cap with yellow ring (PST) tubes are not acceptable.
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| Stability: |
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Ambient: 2 hours on cells
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Interpretation
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Interpretation:
Generally effective therapeutic concentration: 10-20 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 instrument detection limit of 0.4 mcg/mL. Peak serum concentrations can be seen 2-4 hours after ingestion. After oral administration the half-life is about 22 hours, although 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.
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Diphenylhydantoin Dilantin
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