How is carbon monoxide poisoning diagnosed?

Diagnosing carbon monoxide poisoning is very difficult. It's not just homeowners who may find it difficult to diagnose. Carbon monoxide poisoning is also very challenging for health professionals to diagnose.

A research project carried out by St George’s, Epsom and St Helier University Hospitals, University of Surrey and Frimley Health Foundation Trust has sought to reduce misdiagnosis so that patients can be treated appropriately and not sent back to the site of exposure before it is safe to do so.

Read more about this study here.

To understand why it's so difficult to diagnose, it's useful to understand what happens when you are exposed to carbon monoxide.

How does carbon monoxide poison you?

We breathe in carbon monoxide like normal air with no irritation to our noses or throats. Unlike natural gas or LP gas, which have a characteristic odour added to them to alert you, carbon monoxide has no fumes and no colour.

After carbon monoxide has been inhaled, it bonds with our blood cells with greater affinity than oxygen molecules, preventing our organs getting the oxygen they need.

Carboxyhaemoglobin (COHb) is formed by the binding of carbon monoxide to haemoglobin. The formation of COHb reduces the number of blood cells available to transport oxygen.

In short, carboxyhaemoglobin reduces the carrying capacity of haemoglobin for oxygen, which reduces the amount of oxygen carried round the body and results in hypoxia.

Hypoxia is when there is a reduced amount of oxygen reaching tissues in the body.

It is the low levels of oxygen in the body tissues from the hypoxia, which causes the symptoms like confusion, restlessness, difficulty breathing, rapid heart rate and bluish skin.

After exposure, carboxyhaemoglobin levels quickly reduce as the patient breathes fresh air or oxygen. This means by the time a patient reaches the hospital, there is often no trace of the poison to be found.

This offers very few clues to medical professionals that carbon monoxide poisoning may be the cause of the symptoms and so it is often overlooked.

How is carbon monoxide poisoning confirmed?

There is no reliable biomarker to aid the diagnosis of carbon monoxide poisoning. This means that it's hard to prove/diagnose should you suspect you've been exposed to carbon monoxide.

This is because as soon as you leave the source of the exposure, for example, your home to visit your GP or hospital, the carbon monoxide begins to leave your body. It's expelled by breathing fresh air.

When the medical professional takes your blood to check for levels of carboxyhaemoglobin (COhB), the reading will not provide an accurate representation of what may have occurred, if it is taken some time after the patient has left the source of the exposure.

This is a critical area of research. Given the limitations of carboxyhaemoglobin, a new biomarker must be found to make diagnosis easier, ensure that those who need treatment receive it, and ensure that all cases of carbon monoxide exposure are identified.

Then we would have a true picture of the scale of carbon monoxide poisoning.

This may include the development of new techniques, improved patient pathways, and improvements in the application of technology.

Read more about biomarkers and the research we are funding to find a reliable one for carbon monoxide exposure here.

What are the symptoms?

The symptoms of carbon monoxide poisoning are variable and nonspecific, often confused with many other causes.

The most common symptoms of carbon monoxide poisoning are.

  • Headache
  • Dizziness
  • Weakness
  • Upset stomach
  • Vomiting
  • Chest pain
  • Confusion

If you suspect you might have CO poisoning, please visit the NHS website immediately - Here

Carbon monoxide symptoms are often described as “flu-like.”

People who are sleeping or who have been drinking alcohol can die from carbon monoxide poisoning before ever having symptoms.

What is COMA?

For medical professionals, there are a number of red flags which they might look for to help confirm a diagnosis of carbon monoxide poisoning. They can be remembered by using the acronym COMA.

  • Co-habitees - is anyone else in the house affected (including pets)? If there are multiple patients coming from the same location who have similar complaints, then this might be a red flag for carbon monoxide poisoning.
  • Outdoors - do the symptoms improve when out of the house, building or office? If the answer is yes, then it’s possible there is a carbon monoxide leak in the house, building or office.
  • Maintenance - are the heating and cooking appliances properly maintained? If the answer is no, then it is possible that an appliance has caused carbon monoxide poisoning.
  • Alarm - is there a carbon monoxide alarm present and has it been activated?

There are also two other indicators that medical professionals should look for when assessing for carbon monoxide exposure. These are outlined below.

  • No fever - carbon monoxide poisoning does not cause a temperature, so if a patient is presenting with a number of flu-like symptoms, but does not have a fever, then this might be a red flag for carbon monoxide poisoning.

  • History of exposure - If the patient has a history of carbon monoxide exposure this might be a red flag for another incidence of carbon monoxide poisoning.

However, even if carbon monoxide exposure is suspected, it is very difficult for medical professionals to confirm the diagnosis.

Useful links

The UK Government website - Guidance on Carbon monoxide: toxicological overview -

Below Professor Heather Jarman speaks about the challenges faced by clinicians when faced with a possible carbon monoxide poisoning diagnosis.