A 55-year-old man is evaluated in the medical ward on hospital day 5. He was admitted for … — USMLE Step 3

A 55-year-old man is evaluated in the medical ward on hospital day 5. He was admitted for a severe exacerbation of ulcerative colitis. His past medical history includes focal impaired awareness seizures, for which he has taken oral phenytoin 300 mg daily for the past 4 years without recent breakthrough seizures.

During the hospitalization, the following laboratory results and clinical notes were documented:

Time	Albumin	Total Phenytoin	Clinical Action Taken
Hospital Day 1	2.4 g/dL	8.5 mcg/mL	Phenytoin dose increased to 400 mg daily due to subtherapeutic total level.
Hospital Day 5	1.8 g/dL	13.0 mcg/mL	Pending evaluation.

*(Reference ranges: Albumin 3.5-5.0 g/dL; Total Phenytoin 10-20 mcg/mL)*

Today, the patient is lethargic and speaks with slurred speech. Neurologic examination reveals prominent horizontal nystagmus on lateral gaze and severe truncal ataxia when he is assisted to a sitting position. Renal and hepatic function panels are within normal limits.

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

Obtain a serum free phenytoin concentration and decrease the phenytoin dosageCevap
B
Maintain the current phenytoin dosage as the total level is now within the therapeutic range
C
Add valproic acid to the regimen to optimize antiepileptic therapy in a critically ill patient
D
Obtain an urgent magnetic resonance imaging of the brain to evaluate for posterior circulation stroke
E
Discontinue phenytoin and administer an intravenous loading dose of fosphenytoin

Cevap

Obtain a serum free phenytoin concentration and decrease the phenytoin dosage.

The patient is exhibiting classic signs of acute phenytoin toxicity (nystagmus, ataxia, dysarthria, and lethargy) triggered by an unwarranted dose increase on hospital day 1. Phenytoin is approximately 90% bound to serum albumin. When a patient has severe hypoalbuminemia (e.g.,

1.8\ g/dL

in this patient with a severe ulcerative colitis flare), there are fewer binding sites available. Consequently, the fraction of unbound, pharmacologically active (and potentially toxic) phenytoin increases significantly. While the measured *total* phenytoin (

13.0\ \mu g/mL

) appears to be within the therapeutic reference range (

10-20\ \mu g/mL

), the physiologically active *free* concentration is dangerously high. The corrected total phenytoin can be estimated using the Winter-Tozer formula:

Corrected\ Phenytoin = \frac{Measured\ Total\ Phenytoin}{(0.2 \times Albumin) + 0.1}

. On hospital day 5, this yields

\frac{13.0}{(0.2 \times 1.8) + 0.1} \approx 28.3\ \mu g/mL

, which is highly toxic. The correct management is to definitively confirm toxicity by measuring the free phenytoin concentration and to decrease or hold the dosage.

Adım Adım Çözüm

Evaluate the patient's neurologic symptoms.

Lethargy, slurred speech, horizontal nystagmus, and severe truncal ataxia are identified.

These are classic, dose-dependent signs of acute phenytoin toxicity.

Analyze the laboratory data in the context of the patient's presentation.

The total phenytoin level is

13.0\ \mu g/mL

(normal), but albumin is severely depressed at

1.8\ g/dL

Phenytoin is highly protein-bound (approximately 90%). In hypoalbuminemia, total levels misrepresent the actual physiological state.

Calculate the corrected total phenytoin using the Winter-Tozer formula.

Day 5 corrected level:

\frac{13.0}{(0.2 \times 1.8) + 0.1} = \frac{13.0}{0.46} \approx 28.3\ \mu g/mL

Correcting the total level reveals that the physiologically active concentration is well above the upper therapeutic limit of

20\ \mu g/mL

Determine the therapeutic intervention.

The phenytoin dose must be decreased, guided by obtaining a free (unbound) phenytoin level.

The initial dose increase on Day 1 was a medical error caused by misinterpreting a low total level that was actually therapeutic when corrected for albumin. Toxicity must be reversed.

Anahtar Kavram

Therapeutic monitoring of highly protein-bound drugs (like phenytoin) requires measuring free drug levels or mathematically correcting total levels in the setting of significant hypoalbuminemia to prevent iatrogenic toxicity.

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