Blood Gas
Carboxyhaemoglobin
Carboxyhaemoglobin (COHb) is the fraction of haemoglobin bound to carbon monoxide instead of oxygen, reported as a percentage of total haemoglobin by CO-oximetry on a blood gas analyser. Raised levels reduce the blood's oxygen-carrying capacity and are the principal marker used when carbon monoxide exposure or poisoning is suspected.
Why it is measured
It is measured to detect and quantify carbon monoxide exposure, to support triage of smoke-inhalation and suspected poisoning, and to explain a low blood oxygen content when a standard pulse oximeter reads deceptively normal. Smoking status strongly affects the value, so the result is always read against the patient's history.
| Typical range | Adults, non-smokers: about 0.5 to 1.5% of total haemoglobin (some methods quote up to 2 to 3%). Habitual smokers run higher, commonly about 3 to 10%, and occasionally above this in heavy smokers. A COHb above roughly 5% in a non-smoker, or above about 10% in a smoker, is often used as a threshold suggesting carbon monoxide exposure. Reported as a percentage (FCOHb); ranges vary by method and analyser. |
|---|---|
| Sample | Heparinised (anticoagulated) arterial or venous whole blood, analysed promptly by CO-oximetry; venous samples are frequently acceptable because COHb is a property of circulating haemoglobin. Handheld pulse CO-oximeters estimate it non-invasively (reported as SpCO) via a finger sensor. |
| Turnaround | About 35 to 60 seconds on a cartridge blood gas / CO-oximetry analyser once the sample is loaded. Non-invasive pulse CO-oximetry provides a continuous, near-immediate SpCO reading. |
Point of care devices that report it
- Radiometer ABL90 FLEX / ABL90 FLEX PLUS (CO-oximetry module, FCOHb)
- Werfen GEM Premier 5000 (CO-oximetry, COHb)
- Siemens Healthineers RAPIDPoint 500 (CO-oximetry, COHb)
- Masimo Rad-57 Pulse CO-Oximeter (non-invasive SpCO via finger sensor)
Questions, answered
Why can someone with carbon monoxide exposure still show a normal SpO2 on a standard pulse oximeter?
Conventional two-wavelength pulse oximeters cannot tell carboxyhaemoglobin apart from oxyhaemoglobin, so SpO2 can read falsely reassuring. To detect raised COHb you need CO-oximetry on a blood gas analyser or a dedicated pulse CO-oximeter that measures SpCO. This is an educational point, not a substitute for clinical assessment.
Do smokers normally have higher carboxyhaemoglobin than non-smokers?
Yes. Tobacco smoke contains carbon monoxide, so habitual smokers commonly carry a higher baseline COHb. Laboratories interpret results against smoking history and many apply separate indicative ranges for smokers and non-smokers.
Can carboxyhaemoglobin be measured from venous blood, or is an arterial sample essential?
Because COHb reflects haemoglobin already circulating in the blood, the fraction is similar in arterial and venous samples, so venous blood is often acceptable for assessing carbon monoxide exposure. The sample type used should follow local protocols and the analyser's labelling.