Phenytoin, serum
Clinical Background:
Phenytoin (5,5 diphenylhydantoin) is one of the most widely prescribed drugs for the treatment of seizure disorders. It is considered one of the drugs of choice for generalised major motor (grand m...
Phenytoin (5,5 diphenylhydantoin) is one of the most widely prescribed drugs for the treatment of seizure disorders. It is considered one of the drugs of choice for generalised major motor (grand mal epilepsy) and focal seizures. Strong correlation has been observed between serum levels and both therapeutic and toxic effects. In most instances, effective seizure control is observed with serum levels between 10 mg/L and 20 mg/L. Toxic effects are often associated with levels above 25 mg/L.
The primary pathway for phenytoin elimination (90%) involves a metabolic conversion to 5(4 hydroxyphenyl) 5 phenylhydantoin (HPPH) which is a saturable process that could lead to toxic serum levels with a small increase in dosage. Because both metabolism and clearance are highly variable among individuals, a close monitoring of serum levels is appropriate for virtually every patient undergoing phenytoin therapy.
The half life of phenytoin in adults is 9-22 hours after a single dose and 20-40 hours during chronic therapy. There is a shorter half-life in children and infants of 7.5 hours.
Test Documents
Test Details
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Discipline:
Biochemistry
Biochemistry
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Specimen Container Adult:
Serum (SST or plain tube)
Serum (SST or plain tube)
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Specimen Container Paediatric:
Serum (SST or plain tube)
Serum (SST or plain tube)
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Minimum Volume Adult:
1 mL blood
1 mL blood
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Minimum Volume Paediatric:
0.5 mL blood
0.5 mL blood
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Special Requirement:
Trough sample
Trough sample
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Sample Stability:
Unseparated sample: 2 days
Separated sample: 2 days at 15-25°C, 1 month at 2-8°C, 5 months at -20°CUnseparated sample: 2 days
Separated sample: 2 days at 15-25°C, 1 month at 2-8°C, 5 months at -20°C -
Reference Ranges:
Adults: 5 - 20 mg/L
Refer to NuTH "Guideline for the use of injectable phenytoin in adult patients" to calculate the adjusted phenytoin concentration.
Adults: 5 - 20 mg/L
Refer to NuTH "Guideline for the use of injectable phenytoin in adult patients" to calculate the adjusted phenytoin concentration.
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Other Info:
Lithium heparin sample also acceptable
Lithium heparin sample also acceptable
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Routine Contact Name:
Duty Biochemist
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Routine Telephone:
Freeman: 0191 244 8889
RVI: 0191 282 9719Freeman: 0191 244 8889
RVI: 0191 282 9719 -
Routine Email: