Carboxyhemoglobin: Hemoglobin that has carbon monoxide instead of the normal oxygen bound to it. Carbon monoxide has a much great affinity than oxygen for hemoglobin. Carboxyhemoglobin is formed in carbon monoxide poisoning. The source of the carbon monoxide may be exhaust (such as from a car, truck, boat or generator), smoke from a fire, or tobacco smoke. The level of carboxyhemoglobin is a measure of the degree of carbon monoxide exposure.

The presence of carboxyhemoglobin in the blood is called carboxyhemoglobinemia. Carboxyhemoglobin is also known as carbonmonoxyhemoglobin and carbon monoxide hemoglobin.