An 58-year-old male resident of an assisted living facility is evaluated in the emergency … — USMLE Step 3

An 58-year-old male resident of an assisted living facility is evaluated in the emergency department due to a two-day history of frequent falls and worsening gait unsteadiness. Staff members also note that he has become increasingly confused. His medical history includes post-traumatic epilepsy, heart failure, and decompensated cirrhosis. His current medications are phenytoin, spironolactone, furosemide, and lactulose. He reports adhering to his regimen and has two to three soft bowel movements daily. On physical examination, he is alert but disoriented to time and place. Neurologic evaluation reveals prominent horizontal nystagmus on bilateral lateral gaze and a broad-based, unsteady gait. Asterixis is absent.

Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Sodium	$136\ \text{mEq/L}$	$136-145\ \text{mEq/L}$
Creatinine	$0.9\ \text{mg/dL}$	$0.7-1.3\ \text{mg/dL}$
Total bilirubin	$1.8\ \text{mg/dL}$	$0.1-1.2\ \text{mg/dL}$
Albumin	$1.9\ \text{g/dL}$	$3.5-5.0\ \text{g/dL}$
Phenytoin, total	$13\ \mu\text{g/mL}$	$10-20\ \mu\text{g/mL}$

Which of the following is the most appropriate next step in the monitoring and management of this patient?

A
Order a non-contrast computed tomography scan of the head
B
Increase the daily dosage of phenytoin
Obtain a serum free phenytoin concentrationAnswer
D
Add levetiracetam for adjunctive seizure control
E
Discontinue phenytoin and initiate valproate therapy

Answer

Obtain a serum free phenytoin concentration

The correct answer requires recognizing the pharmacokinetic relationship between serum albumin and highly protein-bound medications. The patient exhibits classic, dose-dependent signs of phenytoin toxicity, which include horizontal nystagmus, ataxia, and altered mentation (confusion). Phenytoin is approximately

90\%

bound to serum albumin. In states of significant hypoalbuminemia (such as advanced cirrhosis or nephrotic syndrome), there are fewer binding sites available. This results in a higher proportion of free, pharmacologically active phenytoin. Because standard laboratory assays measure *total* phenytoin (both the albumin-bound and the free unbound fractions), a total level that appears to be within the standard therapeutic range (

10-20\ \mu\text{g/mL}

) can actually harbor a toxic concentration of the active free drug. The appropriate management step is to measure the serum free phenytoin level to accurately assess the extent of toxicity before making empirical dose adjustments or altering the medication regimen.

Step-by-Step Solution

Analyze the patient's clinical presentation.

The patient presents with confusion, horizontal nystagmus, and ataxia.

These are the classic dose-dependent neurologic manifestations of phenytoin toxicity.

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

The total serum phenytoin level is

13\ \mu\text{g/mL}

(within the normal reference range), but the serum albumin is markedly low at

1.9\ \text{g/dL}

The patient's advanced cirrhosis has led to synthetic dysfunction and severe hypoalbuminemia.

Apply pharmacokinetic principles of monitoring to determine the next step.

Identify that phenytoin is extensively protein-bound (

~90\%

Because standard laboratory assays measure total phenytoin (bound + free), a normal total level in a patient with low albumin masks a disproportionately high and toxic level of free (active) phenytoin. A free phenytoin level must be measured to confirm the toxicity and guide dose adjustments.

Key Concept

In patients with marked hypoalbuminemia, highly protein-bound drugs like phenytoin have a larger unbound (active) fraction, meaning patients can experience severe clinical toxicity even when the total serum drug concentration appears within the normal therapeutic range.

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