What is the therapeutic serum level of phenytoin in treatment of seizures?

Appeared in: DSSSB 13 August 2024

Explanation

The established therapeutic serum concentration for total phenytoin in managing seizures is 10 to 20 mcg/mL.
This range provides the best balance between effective anticonvulsant activity and the avoidance of dose-related toxicity.
Levels below 10 mcg/mL are generally considered subtherapeutic, increasing the patient's risk for breakthrough seizures.
Monitoring is critical because phenytoin follows saturation kinetics, where small dose increases can lead to large, unpredictable jumps in serum levels.

Option A: A serum level of 2-5 mcg/mL is subtherapeutic, meaning it is too low to provide adequate protection against seizures.
Option B: A serum level of 40-80 mcg/mL is significantly toxic and would cause severe adverse effects, such as lethargy, confusion, and potentially coma.
Option C: A serum level of 80-120 mcg/mL is extremely toxic and potentially fatal.

Clinical Relevance

Nursing practice connection: Prioritize focused assessment, early escalation, and real-time monitoring when managing Therapeutic drug monitoring of phenytoin in acute care settings.
Phenytoin has a narrow therapeutic index, making close monitoring of serum levels a critical nursing responsibility to prevent toxicity or loss of seizure control.
Nurses must be vigilant in assessing for early signs of toxicity, such as nystagmus (involuntary eye movements), diplopia (double vision), and ataxia (unsteady gait), even when levels are within the high-normal range.
Phenytoin metabolism becomes saturated at therapeutic doses (zero-order kinetics). This means that even a small increase in the daily dose can cause a disproportionately large increase in the serum level, pushing the patient into toxicity.

How to Approach the Question

First, identify the core of the question: it's asking for a specific, memorized value—the therapeutic range of a common medication.
Recognize that phenytoin is a high-alert antiepileptic drug known for its narrow therapeutic index. This classification signals that its therapeutic range is a crucial piece of information for safe nursing practice.
Recall the standard therapeutic range for phenytoin, which is 10-20 mcg/mL.
Compare this recalled range to the options provided.
Select the option that matches the known therapeutic range (10-20 mcg/mL).

Concept Tested & Keywords

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E6 Pharmacology Katzung 16e p. 650-652

E6 Pharmacology Nursing Lilley 11e Part 1 p. 241-243

Practise the full DSSSB 13 August 2024

Attempt every question from this paper in a timed mock, then review the full solution for each one.