Fatal Potassium Thiocyanate Poisoning Presenting With Falsely Elevated Chloride Concentration

Potassium thiocyanate is a long-discontinued weak antihypertensive and an uncommonly encountered toxic agent. We report a 52-year-old man who presented to the emergency department with a sudden onset of altered mental status, vomiting, and seizures after a suspected toxic ingestion. Clinical laboratory tests showed a falsely elevated serum chloride (>200 mmol/L) with other normal electrolytes, resulting in a significant negative anion gap. An endoscopy showed gastric mucosa edema and hyperemia. Repeated electroencephalograms showed moderate to severe encephalopathy. He remained symptomatic until his death 9 days later. An autopsy found hypoxic-ischemic changes in the brain. Toxicological analysis on antemortem blood collected 93 to 95 hours after apparent ingestion revealed a thiocyanate concentration of 260 μg/mL and cyanide concentration of 0.21 μg/mL. Toxicology on postmortem specimens revealed thiocyanate concentrations of 78 μg/mL in cardiac blood, 75 μg/mL in peripheral blood, 16 μg/g in brain, 49 μg/g in kidney, 37 μg/g in liver, and 21 μg/g in skeletal muscle. Further investigation found an internet order for potassium thiocyanate in the days before ingestion. This case report raises the awareness of thiocyanate as a potentially re-emerging toxin and pseudohalide that can interfere with clinical laboratory testing and present as falsely elevated serum chloride (pseudohyperchloremia).