Lab Tests

What do the results of a carbon monoxide (CO) blood test mean?

A Answers (1)

  • AHealthwise answered

    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.


      Carbon monoxide 1
    • 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 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 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.

    This information does not replace the advice of a doctor. Healthwise disclaims any warranty or liability for your use of this information. To learn more visit

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