Repeat the valproate level to confirm accuracy, as the confusion and lethargy are inconsistent with a therapeutic drug level and likely reflect a lab error or recent dose timing issue.

Repeating the valproate level will likely return the same therapeutic result. The symptoms are caused by hyperammonemia, not a supratherapeutic drug level. Hyperammonemic encephalopathy occurs at therapeutic levels.

Order a comprehensive hepatic function panel, as the mildly elevated LFTs combined with asterixis suggest progressive hepatotoxicity that is the primary cause of the encephalopathic presentation.

LFTs may be mildly elevated in hyperammonemia, but the ammonia level is the diagnostic test. Normal LFTs do not rule out hyperammonemic encephalopathy, and mildly elevated LFTs do not confirm hepatotoxicity as the cause.

Obtain a head CT to evaluate for acute cerebrovascular event, as the sudden onset of confusion and lethargy in a 64-year-old with cardiovascular risk factors suggests stroke as the primary differential.

While stroke is in the differential for acute confusion, the combination of valproate use, asterixis, and lethargy points more specifically to metabolic encephalopathy. The asterixis is the key distinguishing finding.

Order an ammonia level, as valproate-induced hyperammonemic encephalopathy can occur at therapeutic drug levels through urea cycle inhibition and is commonly missed.

This is a high-yield board topic. Valproate-induced hyperammonemic encephalopathy occurs at therapeutic levels because the mechanism is urea cycle inhibition, not dose-dependent hepatotoxicity. Valproate inhibits the urea cycle, causing ammonia accumulation that produces confusion, lethargy, and asterixis even when the valproate level is within range. Asterixis (a flapping tremor of the hands) is a classic sign of metabolic encephalopathy — its presence in a patient on valproate should immediately trigger an ammonia level. Management includes dose reduction or discontinuation, possible L-carnitine supplementation, and considering an alternative mood stabilizer. Coordinate with the medical team if ammonia is markedly elevated.