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Difficulty: HardMonitoring and Treatment Evaluation

A 6868-year-old man is evaluated in the intensive care unit for worsening lethargy and confusion. He was admitted 55 days ago for severe community-acquired pneumonia and subsequently developed acute kidney injury. His past medical history is significant for hypertension, type 2 diabetes mellitus, and a focal seizure disorder for which he takes a stable dose of phenytoin. Currently, his temperature is 37.2∘37.2^\circ C (99.0∘99.0^\circ F), blood pressure is 118/74118/74 mmHg, and pulse is 8888/min. Physical examination reveals prominent horizontal nystagmus, slurred speech, and bilateral intention tremor. Laboratory studies show:

Blood urea nitrogen: 6868 mg/dL
Serum creatinine: 3.83.8 mg/dL
Serum albumin: 2.22.2 g/dL
Total serum phenytoin: 1212 mg/L (therapeutic range: 10−2010-20 mg/L)

Which of the following is the most appropriate next step in the evaluation of this patient?

  1. Measure the unbound serum phenytoin concentrationAnswer
  2. B
    Increase the phenytoin maintenance dose
  3. C
    Obtain an emergent non-contrast computed tomography of the head
  4. D
    Initiate continuous electroencephalography monitoring
  5. E
    Add valproic acid to the anticonvulsant regimen

Answer

Measure the unbound serum phenytoin concentration
The correct action is to measure the unbound (free) serum phenytoin concentration. Phenytoin is approximately 90%90\% bound to plasma proteins, primarily albumin. In patients with severe hypoalbuminemia or uremia (where accumulated endogenous toxins competitively inhibit drug binding), the fraction of unbound, pharmacologically active phenytoin increases significantly. Because the standard serum phenytoin assay measures total (both bound and unbound) drug, the total level may appear to be within the normal therapeutic range (10−2010-20 mg/L) even when the free fraction is dangerously elevated. This patient demonstrates classic signs of phenytoin toxicity, including horizontal nystagmus, ataxia, and lethargy. To accurately evaluate and manage this suspected toxicity, the unbound phenytoin concentration (normal range 1−21-2 mg/L) must be measured.

Step-by-Step Solution

1
Analyze the patient's clinical presentation.
The patient exhibits lethargy, horizontal nystagmus, slurred speech, and intention tremor.
These findings are classic clinical manifestations of cerebellar and vestibular dysfunction associated with phenytoin toxicity.
2
Correlate the physical findings with the laboratory results.
The total serum phenytoin level is 1212 mg/L, which is technically within the normal therapeutic range of 10−2010-20 mg/L. However, the patient has severe hypoalbuminemia (2.22.2 g/dL) and uremia (BUN 6868 mg/dL).
Phenytoin is a highly protein-bound drug. Hypoalbuminemia reduces the total number of binding sites, while uremic toxins competitively displace phenytoin from albumin.
3
Determine the impact of the metabolic derangements on drug pharmacokinetics.
The decreased protein binding leads to a significantly increased fraction of free (unbound), pharmacologically active phenytoin.
The standard 'total' phenytoin assay measures both bound and unbound drug. In this altered state, the total level appears falsely reassuring, while the free level is well into the toxic range.
4
Select the most appropriate monitoring intervention.
The correct action is to measure the unbound (free) serum phenytoin concentration.
This confirms the suspected toxicity and guides subsequent dosage adjustments or the decision to temporarily withhold the drug.

Key Concept

Therapeutic drug monitoring of highly protein-bound medications in states of altered protein binding.
Estimated Time:1m 30s
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Topics

Monitoring and Treatment Evaluation

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