What is the recommended monitoring protocol for patients on Dilantin (phenytoin)?

Recommended Monitoring Protocol for Patients on Dilantin (Phenytoin)

Patients on phenytoin therapy require regular monitoring of drug levels, clinical response, and potential adverse effects to ensure optimal therapeutic outcomes and minimize toxicity.

Initial Monitoring

• Serum Phenytoin Levels:

  • Obtain baseline level 7-10 days after starting therapy or changing dosage 1
  • Target therapeutic range: 10-20 mcg/mL (or 10-20 mg/L) 2, 1
  • Free phenytoin levels (unbound) may be more accurate in certain populations

• Baseline Laboratory Tests:

  • Complete blood count with platelets
  • Liver function tests (AST, ALT, alkaline phosphatase)
  • Renal function tests (BUN, creatinine)
  • Serum albumin (affects phenytoin binding) 3

Ongoing Monitoring Schedule

Serum Drug Level Monitoring

• Routine monitoring: Every 3-6 months in stable patients
• More frequent monitoring (monthly or bimonthly) for:
  • Patients with unstable seizures
  • After dosage adjustments
  • Suspected toxicity
  • Addition or removal of interacting medications
  • Changes in health status affecting drug metabolism

Clinical Monitoring

• Monthly clinical assessment for 2:
  • Seizure frequency and characteristics
  • Neuropsychiatric status
  • Visual acuity and red-green color discrimination (for ocular toxicity)
  • Signs of toxicity: nystagmus, ataxia, slurred speech, confusion
  • Gingival hyperplasia and oral health
  • Skin rash or other cutaneous reactions
  • Hirsutism and coarsening of facial features (with long-term use)

Laboratory Monitoring

• Liver function tests:

  • Baseline and periodically, especially in first 3 months of therapy 2
  • More frequent monitoring in patients with preexisting liver disease

• Complete blood count:

  • Periodically to monitor for hematologic adverse effects

Special Monitoring Considerations

Drug Interactions

• Monitor serum levels when adding or removing medications that:
  • Inhibit phenytoin metabolism (increasing levels): valproate, fluconazole, isoniazid
  • Induce phenytoin metabolism (decreasing levels): rifampin, carbamazepine
  • Compete for protein binding sites

Special Populations

• Patients with hypoalbuminemia (liver disease, malnutrition, elderly):

  • Consider monitoring free (unbound) phenytoin levels
  • Adjust interpretation of total phenytoin levels 3

• Patients with renal impairment:

  • Monitor more frequently for toxicity
  • Consider free phenytoin levels

• Pregnant women:

  • More frequent monitoring due to physiologic changes affecting drug levels

Signs of Toxicity Requiring Immediate Assessment

• Nystagmus (earliest sign, appears at levels >20 mcg/mL)
• Ataxia and slurred speech (levels >30 mcg/mL)
• Lethargy, confusion, seizures (levels >40 mcg/mL)
• Visual disturbances (may indicate ocular toxicity)
• New or worsening rash (may indicate hypersensitivity)

Monitoring After Dosage Changes

• Measure serum levels 7-10 days after any dosage change 1
• Do not adjust dosage at intervals shorter than 7-10 days (time needed to reach steady state) 1
• When changing between different phenytoin formulations or brands, monitor levels closely 1

Pitfalls to Avoid

• Treating the "number" rather than the patient - some patients achieve seizure control with levels below 10 mcg/mL while others require higher levels 4
• Failing to account for non-linear pharmacokinetics - small dose increases may cause disproportionate increases in serum levels
• Overlooking drug interactions that can significantly alter phenytoin levels
• Not recognizing early signs of toxicity (nystagmus, mild ataxia)
• Forgetting to monitor for long-term adverse effects (gingival hyperplasia, osteoporosis, peripheral neuropathy)

By following this comprehensive monitoring protocol, clinicians can optimize phenytoin therapy while minimizing the risk of adverse effects and toxicity.