In an 80‑year‑old nursing home resident with a phenytoin (Dilantin) serum concentration of 7.3 µg/mL while receiving 1100 mg per week, how should the dose be adjusted?

Phenytoin Dose Adjustment in an Elderly Nursing Home Resident

Increase the weekly phenytoin dose to approximately 1400-1500 mg per week (200 mg daily) to achieve a therapeutic serum concentration of 10-20 mcg/mL. 1

Current Clinical Situation

The patient's phenytoin level of 7.3 mcg/mL is subtherapeutic (therapeutic range 10-20 mcg/mL), placing them at risk for breakthrough seizures. 1 The current regimen of 1100 mg weekly translates to approximately 157 mg daily, which is insufficient for this patient.

Dosing Considerations in the Elderly

Age-Related Pharmacokinetic Changes

• Elderly patients demonstrate reduced phenytoin clearance with advancing age. 2 Clearance values decrease by approximately one-third between ages 65 and 85 years, though interindividual variability is substantial. 2

• In patients aged 60-79 years, the maximum metabolic rate (Vmax) is significantly lower (6.0 ± 1.9 mg/kg/day) compared to younger adults aged 20-39 years (7.5 ± 2.2 mg/kg/day). 3

• Despite reduced clearance, elderly nursing home residents often require similar or even slightly higher weight-adjusted doses than younger patients to achieve therapeutic levels. 4 In a study of 387 nursing home residents, the mean daily dose was 4.9 mg/kg with a mean serum concentration of 11.7 mg/L. 4

Critical Dosing Principles

Phenytoin exhibits nonlinear (Michaelis-Menten) pharmacokinetics, meaning small dose increases can produce disproportionately large increases in serum concentrations. 1, 5 This is particularly important when adjusting doses in patients with subtherapeutic levels.

Recommended Dose Adjustment Strategy

Step 1: Calculate the Dose Increase

Using the principle that steady-state levels are typically achieved in 7-10 days, and considering the nonlinear kinetics: 1

• Current dose: 1100 mg/week (157 mg/day)
• Current level: 7.3 mcg/mL
• Target level: 12-15 mcg/mL (mid-therapeutic range)

For this patient, increase the weekly dose by approximately 300 mg (to 1400 mg/week or 200 mg daily). 1 This represents a conservative 27% increase that accounts for nonlinear kinetics while minimizing risk of toxicity in an elderly patient.

Step 2: Monitoring Timeline

• Recheck phenytoin level in 7-10 days after the dose change, as this is the time required to achieve new steady-state concentrations. 1

• Do not make further dose adjustments at intervals shorter than 7-10 days. 1

Step 3: Assess for Dose-Related Adverse Effects

Monitor for ataxia, nystagmus, tremor, and somnolence, which are dose-related adverse effects of phenytoin. 6 These symptoms may appear before serum levels reach toxic ranges in elderly patients.

Special Considerations for Nursing Home Residents

Albumin and Protein Binding

• The unbound (pharmacologically active) fraction of phenytoin increases in elderly patients due to hypoalbuminemia. 2 If albumin is low (<3.5 g/dL), the total phenytoin level may underestimate the true pharmacologic effect.

• Consider using the Winter-Tozer equation to adjust measured concentrations for hypoalbuminemia if albumin levels are available. 7

Drug Interactions

• Review all concurrent medications for potential interactions, as phenytoin is a strong enzymatic inducer. 8

• Phenytoin metabolism can be affected by inhibitors (increasing levels) or inducers (decreasing levels), though this study found no significant differences in nursing home residents based on concomitant medications. 4

Gender Considerations

Women in nursing homes may require slightly higher weight-adjusted doses than men to achieve similar serum concentrations (5.1 vs 4.6 mg/kg). 4

Common Pitfalls to Avoid

• Do not make aggressive dose increases in elderly patients due to the risk of disproportionate serum level increases from nonlinear kinetics. 5, 3

• Do not assume elderly patients need lower absolute doses simply based on age—many require standard or higher doses to achieve therapeutic levels. 4, 9

• Do not check levels too soon after dose adjustments; wait the full 7-10 days for steady state. 1

• Do not overlook nutritional interactions if the patient receives enteral nutrition, as this can decrease phenytoin absorption. 7