A 62-year-old man is evaluated in the intensive care unit for new-onset confusion, dizzine… — USMLE Step 3

A $62$ -year-old man is evaluated in the intensive care unit for new-onset confusion, dizziness, and unsteady gait. He was admitted $5$ days ago for severe community-acquired pneumonia complicated by acute respiratory distress syndrome and acute kidney injury. His medical history is significant for generalized epilepsy, for which he takes phenytoin $300\text{ mg}$ daily. Over the past $24$ hours, he has become increasingly lethargic and complains of double vision. Physical examination reveals prominent bidirectional horizontal nystagmus and severe ataxia.

Laboratory studies show:
- Serum creatinine: $4.2\text{ mg/dL}$ (baseline $1.1\text{ mg/dL}$ )
- Blood urea nitrogen: $78\text{ mg/dL}$
- Serum albumin: $2.2\text{ g/dL}$
- Total serum phenytoin: $14\text{ mcg/mL}$ (therapeutic range: $10\text{-}20\text{ mcg/mL}$ )

Which of the following is the most appropriate next step in evaluating and managing this patient's neurologic symptoms?

Obtain a free serum phenytoin concentrationAnswer
B
Obtain a non-contrast CT scan of the head
C
Maintain the current phenytoin dose and monitor closely
D
Discontinue phenytoin and initiate intravenous valproic acid
E
Perform a lumbar puncture for cerebrospinal fluid analysis

Answer

Obtain a free serum phenytoin concentration.

The correct answer is to obtain a free serum phenytoin concentration. Phenytoin is a highly protein-bound medication, with approximately

90\%

bound to serum albumin under normal conditions. The standard laboratory measurement for phenytoin is a total serum level, which includes both the protein-bound and the unbound (free) drug. Because only the free drug is pharmacologically active and capable of crossing the blood-brain barrier to cause toxicity, evaluating the free level is critical in specific populations. In patients with significant hypoalbuminemia (less binding capacity) or acute kidney injury/uremia (where accumulated uremic toxins competitively displace phenytoin from albumin), the fraction of free phenytoin increases substantially. In such scenarios, a 'normal' total phenytoin level (e.g.,

14\text{ mcg/mL}

) can dangerously mask a toxic free phenytoin concentration. This patient exhibits classic signs of phenytoin toxicity, including altered mental status, horizontal nystagmus, and severe ataxia. Given his renal failure and low albumin, assessing the free phenytoin level is the essential next step to confirm toxicity and guide a required dose reduction.

Step-by-Step Solution

Analyze the patient's clinical presentation.

The patient demonstrates lethargy, bidirectional nystagmus, diplopia, and ataxia.

These are the classic, hallmark signs of acute phenytoin toxicity.

Evaluate the laboratory results in the context of the patient's comorbidities.

The total phenytoin level appears normal (

14\text{ mcg/mL}

), but the patient has severe hypoalbuminemia (

2.2\text{ g/dL}

) and acute kidney injury with uremia (BUN

78\text{ mg/dL}

Severe renal failure and low albumin significantly alter the pharmacokinetics of highly protein-bound drugs.

Determine the pharmacokinetic effect of the comorbidities on the medication.

Hypoalbuminemia provides fewer binding sites, and uremic toxins competitively inhibit drug binding to albumin.

This combination leads to a higher fraction of unbound, pharmacologically active (free) phenytoin in the serum, causing clinical toxicity despite a 'normal' total level.

Select the appropriate management step.

The most appropriate next step is to obtain a free serum phenytoin concentration.

This will definitively quantify the suspected toxicity and appropriately guide subsequent dose reduction.

Key Concept

In patients with hypoalbuminemia or uremia, total serum phenytoin levels may appear falsely normal while the biologically active free fraction is in the toxic range; proper evaluation requires measuring free serum phenytoin levels.

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