Carbon Monoxide (CO)

Skip to the navigation

Test Overview

A carbon monoxide blood test is used to detect carbon monoxide poisoning. Poisoning can happen if you breathe air that contains too much carbon monoxide (CO). This gas has no color, odor, or taste, so you can't tell when you are breathing it. The test measures the amount of hemoglobin in your blood that has bonded with carbon monoxide.

Carbon monoxide can come from any source that burns fuel. Common sources are cars, fireplaces, powerboats, woodstoves, kerosene space heaters, charcoal grills, and gas appliances such as water heaters and ovens. These things usually cause no problems. But if they are not used or installed properly, carbon monoxide may build up in an enclosed space.

When you inhale carbon monoxide, it replaces the oxygen that is normally carried by the hemoglobin in your red blood cells. As a result, your brain and other tissues get less oxygen. This can cause serious symptoms or death.

Why It Is Done

This test may be done if you have been exposed to carbon monoxide 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. Tobacco smoke contains carbon monoxide.

Talk to your doctor if you have any concerns about 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 form(What is a PDF document?).

How It Is Done

The health professional taking a sample of your 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. You can treat this by using a warm compress several times a day.

Bleeding can be a problem for people who have bleeding disorders or take blood-thinning medicines such as aspirin or warfarin (Coumadin). 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 carbon monoxide poisoning. Poisoning can happen if you breathe air that contains too much carbon monoxide. The test measures the amount of hemoglobin that has bonded with carbon monoxide. This is called the carboxyhemoglobin level.

The results are reported as a percentage. They show the amount of carbon monoxide bound to hemoglobin divided by the total amount of hemoglobin (multiplied by 100). The higher the percentage, the greater the risk of having symptoms of carbon monoxide poisoning. A person with values below 10% may not have any symptoms.

These numbers are just a guide. The range for "normal" varies from lab to lab. Your lab may have a different range. Your lab report should show what range your lab uses for "normal." Also, your doctor will evaluate your results based on your health and other factors. So a number that is outside the normal range here may still be normal for you.

Results are usually available right away.

Normal

Carbon monoxide footnote 1

People who don't smoke:

Less than 2% of total hemoglobin

People who smoke:

4%–8% of total hemoglobin

High values

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

Symptoms related to high carbon monoxide values footnote 1
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 usually have fewer red blood cells than men do. So women and children may have more severe symptoms at lower levels.

What Affects the Test

  • If you smoke, you already have some carbon monoxide in your 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

  • Anyone who may have been exposed to carbon monoxide and has symptoms should be tested for carbon monoxide poisoning. For example, you should be tested if you live in a house with an old heating system and you have ongoing headaches.
  • If you think you may have carbon monoxide poisoning, you should leave the place of likely exposure and get oxygen to breathe before you are tested.
  • You may also have other tests, such as an arterial blood gases test and a complete blood count. The blood gases test may be done to find out if you have carbon monoxide poisoning or 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

ByHealthwise Staff
Primary Medical ReviewerAnne C. Poinier, MD - Internal Medicine
Specialist Medical ReviewerR. Steven Tharratt, MD, MPVM, FACP, FCCP - Pulmonology, Critical Care Medicine, Medical Toxicology

Current as ofAugust 7, 2014