Phenytoin

Norm
Adults
Therapeutic concentration 20–80 µmol/l
Toxic concentration more than 80 µmol/l
Preterm Infants Therapeutic concentration 24–56 µmol/l
Free, unbound drug 6–10 µmol/l

Synonyms: Phenhydan.
Serum, plasma EDTA
Cap red or violet

Phenytoin is an antiepileptic drug characterized by nonlinear pharmacokinetics (i.e., its biological effect is not directly related to the dose of the drug taken). Therefore, the drug concentration in the blood cannot be easily predicted, and treatment is solely based on the prescribed dose. A slight increase in dose can lead to a significant increase in drug concentration in the serum. A toxic effect can quickly occur after a slight increase in dose. The half-life in adults is 4–42 hours. The time to reach equilibrium in the body is 8–50 days. Phenytoin concentration monitoring is performed for individuals with epilepsy. The first tests should be done 2–3 weeks after starting treatment. Phenytoin interacts with several other medications: barbiturates, dicoumarol. Phenobarbital can increase or decrease phenytoin concentration in the blood. It is recommended to monitor phenytoin concentration in the blood when changing the dose of another drug for the same patient, for example, phenobarbital. The free drug may increase if phenytoin is also being treated with other medications such as salicylates, phenylbutazone, or thiazides. Then the binding of phenytoin to serum transport proteins decreases, and the concentration of the free, unbound drug may increase. This effect is particularly pronounced when there is uremia. The free drug is more active. Carbamazepine or alcohol can decrease phenytoin concentration.

Source | Handbook of Basic Laboratory Tests | Doctor of Medical Sciences Gintaras Zaleskis