How Carbon Monoxide Poisoning Is Diagnosed

Diagnosing carbon monoxide poisoning is harder than it sounds. In theory, carbon monoxide exposure leads to high levels of carbon monoxide in the bloodstream, and that's the diagnosis. The reality is that carbon monoxide exposure is both concentration (how much carbon monoxide is in the air) and time (how long the patient was breathing it), which means that diagnosing carbon monoxide poisoning is a combination of recognizing signs and symptoms as well as measuring the amount of CO in the bloodstream.

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Self Checks/At-Home Testing

There isn't a self-diagnosis option for carbon monoxide poisoning, but anyone with confusion or a loss of consciousness should have 911 called for them.

You should suspect carbon monoxide poisoning if more than one person in a building with a combustion source (furnace, fireplace, gas appliances, wood-burning stove, etc.) is complaining of headaches and nausea.

If carbon monoxide poisoning is suspected, all occupants of a building should go outside to breathe fresh air, along with calling 911. If you suspect CO poisoning, don't try to drive; call an ambulance.

CO in the Blood

Carbon monoxide (CO) binds to hemoglobin similar to how oxygen does. A significance difference, however, is hemoglobin has 230 times the affinity for CO than it does for oxygen. This means even a small amount of inhaled carbon monoxide will bind to hemoglobin, blocking out oxygen in the process.

Hemoglobin that is attached to CO is called carboxyhemoglobin (COHgb). Measurements of the amount of COHgb in the blood are used to detect carbon monoxide in the blood and determine how severe CO poisoning is.

First Responder Testing

Some first responders have the ability to measure carboxyhemoglobin in the blood using a device called a pulse carbon monoxide oximeter. Specifically, the pulse CO-oximeter measures the saturation of carbon monoxide in the hemoglobin (SpCO). It uses light waves (usually shone through the fingertips) to measure carbon monoxide saturation noninvasively.

Another form of noninvasive measurement uses exhaled air to determine levels of carbon monoxide. Some research has found exhaled CO to be inaccurate as a determinate of carbon monoxide poisoning.

SpCO is not universally measured by all first responders, so history and physical examination is still the gold standard at the scene.

Traditional pulse oximetry, used to measure only whether hemoglobin is saturated with oxgyen or not, is fooled by carbon monoxide poisoning into showing artificially high saturation of oxygen when carboxyhemoglobin is present. This makes it even more important to obtain a good history and physical examination of the patient.

Laboratory Tests

In the hospital, a more invasive but more accurate test is used. It is called blood gas.

Blood gas tests measure the amount of atmospheric gases—usually oxygen and carbon dioxide—in the bloodstream by drawing blood from the arteries. Most other blood tests draw blood from the veins, which is easier and safer for the patient.

Arterial blood gas tests are the standard for oxygen and carbon dioxide because those gases change significantly before and after blood flows through body tissues.

Arterial gases—rather than venous—measure the potential for hemoglobin to supply oxygen and to remove carbon dioxide. Since carbon monoxide is neither used by nor easily removed from the bloodstream, it can be tested through either arterial or venous blood.

Blood gas tests are considered more accurate than pulse CO-oximetry. Even though oximetry is useful for identifying patients at the scene who potentially have carbon monoxide poisoning, blood gases should be obtained to confirm carboxyhemoglobin levels.


Acute carbon monoxide poisoning that comes from high concentrations of carbon monoxide in relatively short periods of exposure is not the only effect of carbon monoxide exposure. Chronic (long-term) carbon monoxide exposure at much lower concentrations may cause tissue damage, especially to the heart and brain.

Even though the levels of carboxyhemoglobin in chronic exposure patients might be lower than in acute patients, there are other ways to identify damage. The most common is to look at the tissues through medical imaging.

Magnetic resonance imaging (MRI) is the best way to examine the brain for potential injury from carbon monoxide poisoning.

Differential Diagnoses

Due to the vagueness of most signs and symptoms associated with carbon monoxide poisoning—nausea, vomiting, headache, fatigue, chest pain—other diagnoses are regularly suspected. A high concentration of carbon monoxide at a patient's home will suggest the possibility of carbon monoxide poisoning, but other causes still have to be ruled out.

The list of differential diagnoses is too broad to identify. Each case is different and should be evaluated based on the patient's presentation, history, and tests.

Frequently Asked Questions

How is carbon monoxide in the body measured?

To determine how much carbon monoxide might be in a person's body, a doctor will measure the levels of carboxyhemoglobin (COHgb) in their blood. This compound is formed when carbon monoxide binds with hemoglobin, the protein in red blood cells that carries oxygen. According to the Centers for Disease Control and Prevention, "an elevated COHgb level of 2% for non-smokers and >9% for smokers strongly supports a diagnosis of CO poisoning."

Is there a way to tell if I've been breathing in carbon monoxide?

You can't self-diagnose CO poisoning, but there are signs and symptoms you can be aware of. These symptoms also can provide a clue to how much carboxyhemoglobin (COHgb) may be in your blood:

  • Levels of 10% to 20%: headache and nausea
  • Levels greater than 20%: muscle weakness, dizziness, trouble concentrating, and impaired judgement
  • Levels greater than 30%: dyspnea (shortness of breath), confusion, and chest pain (if you have have coronary artery disease)
  • Higher levels: syncope (fainting), seizures, and altered consciousness (obtundation)

The damage to the body is cumulative: If COHgb blood levels exceed 60%, hypotension (low blood pressure), coma, respiratory failure, and death may occur.

Is cyanide poisoning diagnosed in the same as CO poisoning is?

Cyanide is a colorless gas that, when inhaled in large quantities, prevents cells in the body from using oxygen, much like carbon monoxide does. If cyanide poisoning is suspected, it can be detected with a whole blood test, but because cyanide is eliminated from the blood quickly, testing must be done right away. Cyanide poisoning may occur along with carbon monoxide poisoning in certain cases, such as a house fire, but it is more challenging to diagnose.

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