A phenytoin level test is a quantitative measurement of the concentration of the drug phenytoin (commercially known as Epanutin) in the blood serum. Phenytoin is an anticonvulsant (epileptic) and cell membrane stabilizer. It has a non-linear metabolism (bile excretion), meaning that even a small increase in dose can lead to a large and unexpected rise in its blood level, making it susceptible to toxicity. Therefore, regular monitoring of its levels is essential.