Know the basics

What is Carbon monoxide blood test?

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 is Carbon monoxide blood test performed?

You may need this test if your doctor suspects that you have CO poisoning. Symptoms of carbon monoxide poisoning include:

  • Headache
  • Nausea
  • Dizziness
  • Weakness
  • Diarrhea
  • “Cherry red” skin and lips

Severe poisoning can produce nerve system symptoms, such as:

  • Seizures
  • Coma

Carbon monoxide poisoning can be more difficult to identify in very young children than in adults. For example, a child may merely appear fussy and not want to eat.

You may also have this test if you have been exposed to CO, especially if you inhaled smoke during a fire. You may also have this test if you’ve been near a car that’s had its engine running in an enclosed space for an extended amount of time.

Things to know before

What should I know before receiving Carbon monoxide blood test?

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.

Know what happens

How to prepare for Carbon monoxide blood test?

You do not need to prepare for this test. But you must not smoke before you have this test and be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don’t need a prescription and any illicit drugs you may use.

What happens during Carbon monoxide blood test?

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.

What happens after Carbon monoxide blood test?

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.

Your doctor may also order these tests:

  • Toxicology screen
  • Chest X-ray
  • Pregnancy test in women, because CO exposure puts the fetus at high risk for problems

Your doctor may also order an MRI scan if you also have symptoms of nervous system problems.

If you have any questions about the Carbon monoxide blood test, please consult with your doctor to better understand your instructions.

Understand the results

What do my results mean?

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 is the percentage, the greater is 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.


Carbon monoxide
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 values
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.

Depending on the laboratory and hospital, the normal range for Carbon monoxide blood test may vary. Please discuss with your doctor any questions you may have about your test results.

Hello Health Group does not provide medical advice, diagnosis or treatment.

Review Date: January 4, 2017 | Last Modified: January 4, 2017

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