Interference in glucose and other clinical chemistry assays by thiocyanate and cyanide in a patient treated with nitroprusside

A 3-year-old patient treated with nitroprusside for congestive heart failure had 6.5 mmol/L thiocyanate (toxic, >1.5 mmol/L) and 110 micromol/L cyanide (toxic, >5 micromol/L) present in her blood. At this time a whole-blood glucose concentration assayed on the Nova Stat Profile 5 Plus (Stat Profile) was 25.1 mmol/L. Plasma from that specimen analyzed on a Kodak Ektachem 700 analyzer (E700) indicated 5.2 mmol/L glucose. We investigated the potential interference of dissolved thiocyanate or cyanide on glucose and other routine assays. Toxic concentrations of thiocyanate increased Stat Profile glucose values and E700 total calcium, chloride, and creatinine values. Stat Profile ionized calcium values were decreased by toxic concentrations of thiocyanate. Cyanide (100 micromol/L) decreased alanine aminotransferase activity measured on the E700. Interference with the Stat Profile glucose assay may have been caused by thiocyanate oxidation at the glucose electrode.