Ferrocyanide ingestion may cause false positives in cyanide determination.

The authors present a patient who ingested a cyanide containing solution and arrived at the hospital without any clinical evidence of intoxication but an elevated blood cyanide level. The authors explain this discrepancy with the following hypotheses: 1) the patient ingested cyanide as an iron-chelated complex; and 2) the sulfuric acid used in the standard microdiffusion technique released cyanide from its iron-bound state to result in the observed elevated blood cyanide. Through a series of in vitro analyses, the authors demonstrate the following: 1) the ingested solution tested positive for cyanide with the sulfuric acid technique and negative for cyanide with acetic acid; 2) the presence of a ferrous salt in the ingested product by a colorimetric redox titration technique; and 3) release of a small fraction of the total cyanide from ferrocyanide by the sulfuric acid technique. The authors conclude: 1) the patient ingested potassium ferrocyanide; and 2) the strong acid used in the cyanide microdiffusion assay will liberate cyanide that is chelated to iron to yield false positive results.