Haemoglobin is protein in redblood cells which carries oxygen. Carboxy-Haemoglobin is the proportion of normal haemoglobin with carbon monoxide (CO) bound to it. It is caused by deliberate self-poisoning with motor vehicle exhaust fumes, and to a much lesser extent by cigarette smoking and air pollution.
How the Test is Performed
Carboxy-Haemoglobin (CO-Hb) is a blood test that may be taken from a vein, or from an artery. Modern blood-gas analysers can produce a result within a few minutes.
Medical Conditions and Symptoms
Haemoglobin is a protein in red blood cells which carries oxygen. Carboxy-Haemoglobin is the proportion of normal haemoglobin with carbon monoxide (CO) bound to it.
Because a small amount of carbon monoxide is produced by normal metabolic processes, even normal healthy people have a small amount of Carboxy-Haemoglobin in their blood. Smokers have higher levels of Carboxy-Haemoglobin (COHb) in their blood than non-smokers.
Carbon Monoxide is a colourless, odourless gas. 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.
The commonest symptoms of CO poisoning include headache, nausea, weakness, blurred vision and dizziness. Severe poisoning may lead to drowsiness, loss of consciousness or even deep coma. 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.
Test Results Explained
Carboxy-Haemoglobin 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 “offloaded” 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 (Diving Specialist), the practical difficulties of administering HBO means that many people are unable to receive this therapy.
Related Specialists
- General Practitioner (GP)
- Occupational Health Physician
- Toxicologist
- Emergency Physician
- Intensivist
- General Physician
- Hyperbaric Medicine / Diving Physician
Related Procedures
- Blood Test (venesection)
Related Tests
- Glucose
- Drug Levels
- Urea & Electrolytes
- Liver Function Tests
- Full Blood Count
- Chest X-Ray (CXR)
- Electrocardiogram (ECG)
Also Known As
- HbCO
- Hb-CO
- Carboxyhaemoglobin (carboxyhemoglobin)
- Carbon Monoxide levels
