Cyanide Toxicity Workup

Updated: Sep 12, 2023
  • Author: Inna Leybell, MD; Chief Editor: Michael A Miller, MD  more...
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Approach Considerations

The workup in patients with cyanide exposure may include the studies discussed below.

Arterial and venous blood gases

Cyanide toxicity is characterized by a normal arterial oxygen tension and an abnormally high venous oxygen tension, resulting in a decreased arteriovenous oxygen difference (< 10%). A high-anion-gap metabolic acidosis is a hallmark of significant cyanide toxicity. [10, 27] Apnea may result in combined metabolic and respiratory acidosis.

Blood lactate level

Elevation in the blood lactate level is a sensitive marker for cyanide toxicity. A plasma lactate concentration of greater than 10 mmol/L in smoke inhalation or greater than 6 mmol/L after reported or strongly suspected pure cyanide poisoning suggests significant cyanide exposure. [28]

Red blood cell or plasma cyanide concentration

Cyanide blood concentrations are not generally available in time to aid in the treatment of acute poisoning, but may provide subsequent confirmation. In cyanogen exposures, these tests provide documentation for therapeutic use, which may last several days.

The preferred test is a red blood cell cyanide concentration. With this method, mild toxicity is observed at concentrations of 0.5-1.0 μg/mL. Concentrations of 2.5 μg/mL and higher are associated with coma, seizures, and death. Blood cyanide concentrations may artificially increase after sodium nitrite (antidote) administration, because of in vitro release of cyanide from cyanomethemoglobin during the analytical procedure by strong acid used in analysis.

Carboxyhemoglobin level or blood carbon monoxide concentration

Carboxyhemoglobin (HbCO) level (by co-oximetry) or blood carbon monoxide concentration (by infrared spectroscopy) may be obtained in patients with smoke inhalation to rule out concurrent exposure. HbCO measurements may be artificially elevated in blood samples drawn after hydroxocobalamin administration. [29]

Methemoglobin level

A methemoglobin level is especially important in cyanotic patients. The presence of methemoglobin suggests that little or no free cyanide is available for binding, because methemoglobin vigorously binds cyanide to form cyanomethemoglobin (which is not measured as methemoglobin).

Methemoglobin concentrations provide a guide for continued therapy after the use of methemoglobin-inducing antidotes, such as sodium nitrite. Elevated levels of methemoglobin (> 10%) indicate that further nitrite therapy is not indicated and, in fact, may be dangerous.

Electrocardiogram (ECG)

On ECG, nonspecific findings predominate. Abnormalities may include the following [30] :

  • Sinus bradycardia or tachycardia
  • Atrioventricular blocks
  • Supraventricular or ventricular arrhythmias
  • Ischemic changes

In some cases, shortening of the ST segment with eventual fusion of the T wave into the QRS complex has been observed.


No imaging studies are indicated acutely for cyanide exposure, but magnetic resonance imaging (MRI) may be useful during the evaluation of postexposure neurologic sequelae.

Fluorescein staining and slit-lamp examination of the eyes may be necessary following decontamination, to assess corneal integrity.