What is carboxyhaemoglobin?

Carboxyhaemoglobin (CO-Hb) is the proportion of normal haemoglobin with carbon monoxide (CO) bound to it. Higher levels of carboxyhaemoglobin is commonly caused by deliberate self-poisoning with carbon monoxide, such as motor vehicle exhaust fumes, and to a much lesser extent by cigarette smoking and air pollution.

What is a carboxyhaemoglobin test?

Carboxyhaemoglobin is a blood test used to diagnose suspected carbon monoxide poisoning, as well as montoring its treatment.

Blood is taken for the test from a vein, or from an artery. Modern blood-gas analysers can produce a result within a few minutes.

What is carbon monoxide poisoning?

A small amount of carbon monoxide is produced by normal metabolic processes, even normal healthy people have a small amount of carboxyhaemoglobin in their blood. Smokers have higher levels of carboxyhaemoglobin in their blood than non-smokers.

Because of this, CO poisoning can occur in an occupational setting, where incomplete combustion may release the gas, or at home, due to a faulty heater or radiator, without any warning signs.

Deliberate attempts at self-poisoning with motor vehicle exhaust fumes can also lead to CO toxicity, however the use of catalytic converters in most modern vehicles dramatically decreases their carbon monoxide output.

CO poisoning decreases the amount of oxygen that the blood can carry, therefore people with conditions of the blood, cardiovascular or respiratory systems are more prone to the symptoms of their underlying conditions when poisoned by carbon monoxide.

For example, people with anaemia (low haemoglobin or blood count) or chronic obstructive airways disease are more likely to become short of breath, and people with angina may develop chest pain due to CO poisoning.

Book your health appointments online

Find and instantly book your next health appointment with Healthengine

Find health practitioners

What are the symptoms of carbon monoxide poisoning?

The most common symptoms of CO poisoning include:

  • Headache
  • Nausea
  • Weakness
  • Blurred vision
  • Dizziness

Severe poisoning may lead to drowsiness, loss of consciousness or even deep coma.

Carboxyhaemoglobin test results explained

Carboxyhaemoglobin levels are often quoted as being predictive of certain symptoms. While it is true that higher levels are more likely to result in more severe symptoms, the blood levels are only a rough guide to the degree of CO exposure.

Blood levels show how much CO is being transported by the red blood cells at any one point in time, but they do not reflect how much has already been transported and ‘off-loaded’ into the tissues, where the damage is being done.

Many hyperbaric facilities still quote a specific percentage of carboxyhaemoglobin as a criterion to consider treatment with Hyperbaric oxygen (HBO).

This area of toxicology is highly controversial, as the research studies show conflicting results as to the benefits of HBO treatment. It is important to remember that although this treatment may be considered, or discussed with a hyperbaric medicine physician, the practical difficulties of administering HBO means that many people are unable to receive this therapy.

Related specialists

Related procedures

  • Blood Test (venesection)

Related tests

Also known as

  • HbCO
  • Hb-CO
  • Carbon monixide levels



A: Use HealthEngine to find and book your next GP appointment. Click on the following locations to find a GP clinic in your state or territory.

This article is for informational purposes only and should not be taken as medical advice. If in doubt, HealthEngine recommends consulting with a registered health practitioner.

All content and media on the HealthEngine Blog is created and published online for informational purposes only. It is not intended to be a substitute for professional medical advice and should not be relied on as health or personal advice. Always seek the guidance of your doctor or other qualified health professional with any questions you may have regarding your health or a medical condition. Never disregard the advice of a medical professional, or delay in seeking it because of something you have read on this Website. If you think you may have a medical emergency, call your doctor, go to the nearest hospital emergency department, or call the emergency services immediately.