Skip to Content

(785) 505-5000

Carbon Monoxide (CO)

Test Overview

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 amount is also called the carboxyhemoglobin level.

When a person inhales carbon monoxide, it combines with the red blood cells that normally carry oxygen to the body's tissues and replaces the oxygen that is normally carried in the blood. As a result, less oxygen is carried to the brain and other body tissues. Carbon monoxide can cause severe poisoning and death.

Carbon monoxide is made during burning when there is not enough oxygen present for complete combustion. The main sources of carbon monoxide are engine fumes (such as from cars or boats), fires burning with poor ventilation (such as gas heaters and indoor cooking fires), factories, and smoking tobacco.

Why It Is Done

A carbon monoxide blood test is used to detect poisoning from breathing carbon monoxide. You might have this test if you have been exposed to the gas or if you have unexplained symptoms, such as:

  • Headache, dizziness, or vision problems.
  • Nausea or vomiting.
  • Muscle weakness.
  • Confusion or trouble thinking.
  • Extreme sleepiness.

How To Prepare

Do not smoke before you have this test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean. To help you understand the importance of this test, fill out the medical test information formmedical test information form(What is a PDF document?).

How It Is Done

The health professional drawing blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Put a gauze pad or cotton ball over the needle site as the needle is removed.
  • Put pressure on the site and then put on a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

There is very little chance of a problem from having a blood sample taken from a vein.

  • You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
  • In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
  • Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

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.

References

Citations

  1. Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

Other Works Consulted

  • Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.

Credits

By Healthwise Staff
Primary Medical Reviewer Anne C. Poinier, MD - Internal Medicine
Specialist Medical Reviewer R. Steven Tharratt, MD, MPVM, FACP, FCCP - Pulmonology, Critical Care Medicine, Medical Toxicology
Current as of June 4, 2014

This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use. How this information was developed to help you make better health decisions.

© 1995-2014 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.