Phenytoin (Dilantin) and Levetiracetam (Keppra) Therapeutic Drug Monitoring Frequency
For phenytoin (Dilantin), obtain serum levels at steady-state (5-7 half-lives, approximately 7-10 days after initiation or dose change), then periodically when clinically indicated—not routinely. For levetiracetam (Keppra), routine therapeutic drug monitoring is generally not recommended, as there is no established correlation between serum levels and efficacy or toxicity 1, 2.
Phenytoin (Dilantin) Monitoring
Initial Monitoring
- Obtain levels at steady-state: 5-7 half-lives after treatment initiation, dosage change, or addition/subtraction of another drug 3
- This translates to at least 7-10 days after starting therapy with recommended doses of 300 mg/day 3
- The average half-life is 22 hours (range 7-42 hours), making steady-state timing variable 3
Ongoing Monitoring Indications
Phenytoin levels should be checked when:
- Suspected toxicity: Nystagmus typically appears at 20 mcg/mL, ataxia at 30 mcg/mL, dysarthria and lethargy over 40 mcg/mL 3
- Breakthrough seizures despite apparent therapeutic dosing
- Drug interactions: Phenytoin is a potent CYP450 inducer with numerous interactions 3
- Special populations: Renal/hepatic disease, hypoalbuminemia, pregnancy, elderly patients, or those with CYP2C9/CYP2C19 genetic variants 3
- Wide interpatient variability: Patients with large variations in levels despite standard doses present difficult clinical problems where monitoring is particularly helpful 3
Critical Caveat
Routine monitoring without clinical indication is unjustified. A 1989 study found that 56% of phenytoin levels measured were not justified on pharmacokinetic or clinical grounds, representing unnecessary costs 4. Only order levels when there is a specific clinical or pharmacokinetic reason.
Levetiracetam (Keppra) Monitoring
General Recommendation
Routine therapeutic drug monitoring is NOT recommended for levetiracetam in most patients 1, 2. The drug has:
- Linear, predictable dose-concentration relationships
- Minimal drug-drug interactions
- Broad therapeutic window
- No established correlation between plasma levels and efficacy or toxicity 1, 2
Exceptions: When to Consider Monitoring
Levetiracetam monitoring may be useful in specific populations with altered pharmacokinetics 1:
Pediatric patients: Clearance increased by 30-40% compared to adults—may require higher weight-based doses 1
Elderly patients (>65 years): Clearance decreased by almost 50%—may require dose reduction 1
Renal impairment: Clearance declines linearly with creatinine clearance; highly variable in end-stage renal disease or those on renal replacement therapy 1, 5
Critically ill patients: Augmented clearance may necessitate higher doses 1
Pregnancy: Levels decline as pregnancy progresses due to increased glomerular filtration rate; remain variable postpartum 1
Drug interactions: Phenytoin co-administration increases levetiracetam clearance by 43.1%, potentially requiring dose adjustment 5
Establishing individual therapeutic range: If a specific concentration correlates with seizure control in an individual patient, monitoring that target may be beneficial, particularly before pregnancy 2
Commonly Observed Therapeutic Range
While not validated for routine use, concentrations of 6-18 mg/L (or 20-40 mg/L in some studies) are typically observed during effective therapy 6, 2. However, seizure control and adverse effects occur both within and outside these ranges, limiting their clinical utility 1.
Practical Algorithm
For Phenytoin:
- Check level 7-10 days after initiation or dose change
- Recheck only when: toxicity suspected, breakthrough seizures, drug interactions added, or special populations with altered pharmacokinetics
- Avoid routine monitoring without clinical indication
For Levetiracetam:
- Do NOT routinely monitor in stable patients
- Consider monitoring in: pediatrics, elderly, renal failure, critically ill, pregnant patients, or those on phenytoin
- If monitoring, establish an individual therapeutic target rather than relying on population ranges