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Paediatric Laboratory Medicine

Phenytoin (Total)

Clinical Significance

Phenytoin is one of the most widely prescribed drugs for the treatment of seizure disorders. It is considered one of the drugs of choice for generalized major motor and focal seizures. Strong correlation has been observed between serum levels and both therapeutic and toxic effects. 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. Phenytoin toxicity primarily affects the centrol nervous sytem. Chromic treatment may lead to hyperplasia of the gums, anemia and osteomalacia. 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.

Test Name

Phenytoin (Total)

Alternate Name/ Synonym

Dilantin, Phosphenytoin

Test Code


Drug Class



Therapeutic Drug Monitoring - TDM


Abbott Architect ci4100

External Proficiency Testing


Turn Around Time

Daily (throughout the day)



Specimen Type

Serum, Plasma (Heparin or Citrate)

Minimum Specimen Requirements

150 uL

Optimal Collection Time

Trough: 0-30 minutes prior to dose


4°C (transport with a cool pack if possible)

Approval is not required

CPT Codes


Shipping and Contact Information

The Hospital for Sick Children
Rapid Response Laboratory
170 Elizabeth Street, Room 3642
Toronto, ON
M5G 2G3
Phone: 416-813-7200
Phone: 1-855-381-3212

Therapeutic Range

40-80 umol/L

Critical Values

> 85 umol/L