Phenytoin therapeutic range is 10 to 20 mcg/mL. Level 28 represents toxicity. Classic toxicity findings: nystagmus (the earliest sign), diplopia, ataxia, slurred speech, confusion. Action: HOLD the dose, notify provider for dose adjustment, place on fall precautions, and complete neurologic assessment. Doubling the dose (3) is dangerous; activated charcoal (4) is not appropriate for chronic toxicity (it is used in acute overdose within ~1 hour of ingestion).
In-depth explanation
Phenytoin levels and signs are highly testable: 10-20 therapeutic, 20-30 nystagmus + diplopia + ataxia, 30-40 lethargy, > 40 coma/seizures (paradoxically). The earliest detectable sign is horizontal nystagmus on lateral gaze — assess routinely.
For study reference only. Always follow current clinical guidelines and your institution’s protocols.