From the Guidelines
The therapeutic blood level range for patients on Dilantin (phenytoin) is 10-20 mcg/mL. This range is crucial for achieving seizure control while minimizing toxicity, as supported by the clinical policy for evaluating and managing adult patients presenting to the emergency department with seizures 1. When monitoring phenytoin levels, it's essential to consider the timing of blood samples, ideally at trough (just before the next dose), and to check free phenytoin levels in specific patient populations, such as those with hypoalbuminemia, renal dysfunction, or those taking medications that compete for protein binding, as only the unbound (free) portion is pharmacologically active. Some key points to consider include:
- The therapeutic range for free phenytoin is 1-2 mcg/mL.
- Levels above 20 mcg/mL are associated with an increasing risk of side effects, including nystagmus, ataxia, slurred speech, lethargy, and confusion.
- Phenytoin exhibits non-linear (zero-order) kinetics at therapeutic doses, meaning small dose increases can lead to disproportionately large increases in serum levels, necessitating careful titration and monitoring, especially when approaching the upper therapeutic range. Given the information provided in the clinical policy 1, it is clear that maintaining phenytoin levels within the therapeutic range is critical for effective seizure management and minimizing adverse effects.
From the FDA Drug Label
The clinically effective serum level is usually 10–20 mcg/mL Optimum control without clinical signs of toxicity occurs more often with serum levels between 10 and 20 mcg/mL, although some mild cases of tonic-clonic (grand mal) epilepsy may be controlled with lower serum levels of phenytoin
The therapeutic blood levels for patients on Dilantin (phenytoin) are 10-20 mcg/mL. This range is considered to be the clinically effective serum level for most patients, although some patients may be controlled with lower serum levels 2 2.
From the Research
Therapeutic Blood Levels for Phenytoin
The therapeutic blood levels for patients on Dilantin (phenytoin) are typically considered to be between 10-20 micrograms/ml 3.
Subtherapeutic and Toxic Levels
- Subtherapeutic levels: below 10 micrograms/ml
- Toxic levels: above 20 micrograms/ml 4
Importance of Therapeutic Drug Monitoring
Therapeutic drug monitoring of phenytoin is crucial due to its narrow therapeutic index and large inter-individual differences in pharmacokinetics 5, 3.
Factors Affecting Therapeutic Levels
The frequency of concentrations within the therapeutic ranges decreases in patients using more than one antiepileptic drug 3.
Clinical Relevance
Monitoring phenytoin levels is essential to avoid toxicity and ensure efficacy, as toxic levels can cause adverse effects such as downbeat nystagmus 4, while subtherapeutic levels may lead to inadequate seizure control.