Serum thiocyanate levels

A 78-year-old male experiences a hypertensive emergency and is given sodium nitroprusside intravenously. This patient’s nitric oxide levels can be monitored by which of the following laboratory tests?

Methemoglobin levels
Plasma nitroprusside concentration
Serum thiocyanate levels
Urinary cyanide concentration
Urinary nitroprusside concentration

Correct answer
Serum thiocyanate levels
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correct answer: C

Nitroprusside is non-enzymatically transformed to cyanide to form cyanomethemoglobin. The cyanide is then metabolized to thiocyanate in the plasma. This assay can assess the effectiveness of therapy. Plasma nitroprusside concentration is incorrect because the half life of the drug is usually too short to make this a meaningful test, and the other choices are biochemically and physiologically not feasible.