At a Glance

Cyanide is a rapid-acting poison with a steep dose-response curve. Death can occur within minutes following cyanide exposure. Signs and symptoms of poisoning can vary, depending on the chemical form (i.e., solid, liquid, or gas), route, dose, and speed of intake, as well as patient-specific factors, such as age, gender, overall health, stress level, and weight. The brain is the organ most sensitive to the toxic effects of cyanide, and, thus, impairment of the central nervous system dominates the initial clinical picture.

The scent of burnt almonds on the patient’s breath is a typical symptom of cyanide poisoning, although lack of the scent does not exclude the possibility of cyanide poisoning. The patient may present with lethargy, sudden collapse, coma, dyspnea, tachypnea, tachycardia, and hypotension. Severe poisoning may result in bradypnea, bradycardia, cardiovascular shutdown, and death. The dominant effects result from cyanide’s inhibition of the mitochondrial enzyme cytochrome c oxidase, which disrupts oxygen uptake, adenosine tri-phosphate (ATP) production, and oxidative metabolism and forces anaerobic metabolism. This state results in lactic acidosis, metabolic acidosis with a large anion gap, and reduced arteriovenous oxygen content difference.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

If cyanide poisoning is suspected, precautions must be taken to protect rescuers and providers from cyanide on the patient’s skin and clothing, in exhaled air or vomitus, and/or in the environment. Patient decontamination may be necessary to reduce further exposure. Diagnosis must be based almost entirely on the clinical picture because of the quick onset of cyanide toxicity and the lack of a rapid cyanide test onsite at most hospitals. Patient history may provide important clues for diagnosis if the patient was a victim of an attempted suicide, homicide, accident, or occupational exposure to cyanide.

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Blood cyanide testing is performed for confirmatory or forensic purposes only. A blood concentration up to 0.1 mg/L is considered normal. A cyanide blood concentration greater than 0.2 mg/L is in the toxic threshold, whereas fatal levels usually exceed 1 mg/L. Interpretation of cyanide concentrations cannot be used for clinical management. Blood cyanide level is not a predictable indicator for severity of illness and the body continues to eliminate cyanide as long as the patient is alive.

Other laboratory tests are ordered for monitoring the patient and evaluating complications. Typically, laboratory testing is performed on blood specimens for complete blood count (CBC), glucose, and electrolytes (sodium, potassium, chloride, and bicarbonate). Patients exposed to hydrogen cyanide also receive the following tests: serum lactate, arterial blood gas measurements (ABG); pH, PO2, CO2, and bicarbonate), electrocardiogram (ECG) monitoring, and chest radiography. Results from these tests assist the provider in clinical management of initial interventions and in monitoring cyanide antidotal therapy.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?

Proper specimen collection and storage are critical for accurate measurement of cyanide concentration. Cyanide testing should be performed on whole blood not coagulated. Heparinized vials should be tightly sealed to prevent evaporation of cyanide at the physiological pH. Cyanide testing should commence as quickly as possible as artifactual formation of cyanide may occur in stored specimens.

Antidotes may be used to enhance cyanide excretion. Various forms of antidotes exist in the literature. Laboratory results should be interpreted in the context of the specific antidote given, as these compounds may impact laboratory findings. The recommended therapy for treatment in the United States is a combination of a nitrite and sodium thiosulfate to enhance renal secretion of cyanide.

What Lab Results Are Absolutely Confirmatory?

Whole blood cyanide testing is performed for confirmatory or forensic purposes.

What Confirmatory Tests Should I Request for My Clinical Dx? In addition, what follow-up tests might be useful?

Few victims survive high dose cyanide exposure without aggressive medical support. The first step in treating cyanide poisoning is to recognize the situation. Failure to identify cyanide poisoning can lead to additional casualties. Emergency responders, health care providers, family members, and other bystanders near the patient may become victims if the source of exposure is not contained and the patient not properly decontaminated. The second step involves rapid intervention to avoid or minimize complications. Emergency life-saving procedures may be necessary but must be done with protective measures in place. The third step is to provide supportive care in accordance with the extent of poisoning. The fourth step is to provide cyanide antidotes and to respond to undesirable side effects. This step usually begins without the results of blood cyanide confirmatory tests.

Errors in Test Selection and Interpretation

A cyanide test should not be ordered on a patient who is on nitroprusside therapy, as sodium nitroprusside metabolism involves the production of cyanide. The serum thiocyanate test should be ordered to monitor nitroprusside therapy. Conversely, a thiocyanate test should not be used to confirm the presence of cyanide, as it only detects the thiocyanate metabolite of cyanide.

A pulse oximeter should not be used in place of ABG measurements for a patient on nitrite therapy, as elevated methemoglobin skews oxygen saturation oximeter readings.

Caution must be taken when interpreting blood cyanide test results, as a wide range of values and test platforms exist in the literature. Most therapies were developed using animal models or a small number of case reports in which a person was accidentally exposed as clinical trials are ethically prohibitive. Even fewer reports exist for providing information on cyanide intoxication in pregnant women, fetuses, and children.