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Carbon Monoxide (CO)

Results

A carbon monoxide blood test is used to detect poisoning from breathing carbon monoxide (CO), a colorless, odorless, poisonous gas. The test measures the amount of hemoglobin that has bonded with carbon monoxide. This is also called the carboxyhemoglobin level.

Carbon monoxide results are reported as a percentage: The amount of carbon monoxide bound to hemoglobin is divided by the total amount of hemoglobin (and then multiplied by 100). The higher the percentage, the greater the risk of having symptoms of carbon monoxide poisoning. With values below 10%, a person may not have any symptoms of poisoning.

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

Results are usually available right away.

Normal

Carbon monoxide1
Nonsmokers:

Less than 2% of total hemoglobin

Smokers:

4%–8% of total hemoglobin

High values

High blood carbon monoxide values are caused by carbon monoxide poisoning. Symptoms of carbon monoxide poisoning become more severe as the carbon monoxide levels increase.

Symptoms related to high carbon monoxide values1
Percent of total hemoglobin Symptoms

20%–30%

Headache, nausea, vomiting, and trouble making decisions

30%–40%

Dizziness, muscle weakness, vision problems, confusion, and increased heart rate and breathing rate

50%–60%

Loss of consciousness

Over 60%

Seizures, coma, death

Women and children may have more severe symptoms of carbon monoxide poisoning at lower carbon monoxide levels than men because women and children usually have fewer red blood cells.

What Affects the Test

  • Smokers already have some carbon monoxide in their blood.
  • People who are regularly exposed to car exhaust, such as taxi drivers and traffic police, often have high carbon monoxide levels (8% to 12%).

What To Think About

  • A person with symptoms and possible exposure to carbon monoxide, such as someone who lives in a house with an old heating system and complains of ongoing headaches, should be tested for carbon monoxide poisoning.
  • A person who may have carbon monoxide poisoning should be removed from the place of likely exposure and given oxygen to breathe before being tested.
  • If carbon monoxide poisoning is suspected, other tests (such as arterial blood gases and a complete blood count) may be done. An arterial blood gas (ABG) test may be done to determine whether symptoms are caused by carbon monoxide poisoning or by another disease that causes similar symptoms. See the topics Arterial Blood Gases (ABG) and Complete Blood Count (CBC).

WebMD Medical Reference from Healthwise

Last Updated: March 01, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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