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Interpretive Handbook

Test 8691 :
Cyanide, Blood

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Cyanide (hydrocyanic acid, prussic acid) blocks cellular respiration by binding to and inactivating hemoglobin and enzymes such as cytochrome oxidase having prosthetic groups containing ferric iron (Fe+++). Cyanide is metabolized rapidly by the liver where it is converted to thiocyanate.


Therapy of hypertensive crisis with nitroprusside (Nipride) results in elevated cyanide blood concentrations, because the cyano function is transferred from nitroprusside to hemoglobin in the red cell.


Symptoms of cyanide poisoning include giddiness, hyperpnea, headaches, palpitation, cyanosis, and unconsciousness. Asphyxial convulsions may precede death; death normally ensues within a few minutes to 3 hours, depending upon the dose. As long as the heart continues to beat, there is a chance of saving the patient because effective antidotes are available; treatment with sodium nitrite and sodium thiosulfate can be effective.

Useful For Suggests clinical disorders or settings where the test may be helpful

Monitoring possible exposure to cyanide


Establishing cause of death in cyanide exposure


Not useful for monitoring nitroprusside.

Interpretation Provides information to assist in interpretation of the test results

Blood concentrations in the average population are as high as 0.2 mcg/mL, mostly derived from vegetables such as brussel sprouts. 


Significant contact with cyanide can produce blood concentrations up to 2.0 mcg/mL without side effects.


At concentrations of 2.0 to 4.0 mcg/mL, giddiness, headaches, and hyperpnea are evident.


Concentrations >5.0 mcg/mL are potentially lethal. 


Normal concentration: <0.2 mcg/mL


Toxic concentration: > or =2.0 mcg/mL

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Concentrations as high as 8.0 mcg/mL without symptoms of cyanide toxicity are commonly associated with nitroprusside therapy; one should monitor thiocyanate to determine the potential for intoxication during nitroprusside therapy for hypertensive crisis.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Normal: <0.2 mcg/mL

Toxic concentration: > or =2.0 mcg/mL

Clinical References Provides recommendations for further in-depth reading of a clinical nature

Medical Toxicology, Third edition, Edited by RC Dart. 2004 pp 1162, 1800