Carboxyhemoglobin is a form of blood protein that is bound to carbon monoxide. Levels of this compound are higher in smokers, and in areas with carbon monoxide poisoning. Carbon monoxide is an odorless, colorless gas, so individuals do not always realize when they have been exposed to it. If such poisoning is detected early enough, it can be treated by providing pure oxygen to the patient.
Hemoglobin is the most common protein in red blood cells (RBC). It transports oxygen throughout the body. Carbon monoxide can bind hemoglobin over 200 times more tightly than oxygen, however. Hemoglobin bound to carbon monoxide is known as carboxyhemoglobin. When in this form, hemoglobin can no longer carry oxygen. The body can therefore become deprived of oxygen, and a person can die from carbon monoxide poisoning.
Some of this compound is produced through normal cellular metabolism, but most of it is inhaled, such as by smoking tobacco. Smokers have elevated carboxyhemoglobin levels. Other ways people are exposed to this toxic gas can include being near malfunctioning appliances powered by natural gas, like stoves or furnaces. Burning charcoal indoors or inhaling smoke from fires can be additional sources of carbon monoxide. Also, being in an enclosed space while a car is running is another way of being exposed.
A carboxyhemoglobin test measures the level of this form of hemoglobin in the blood. The normal level reported for non-smoking adults is typically less than 2.3%. People who smoke one to two packs of cigarettes per day have levels ranging from 4-5%, while those who smoke more than two packs each day generally have levels as high as 8 to 9%. Toxic levels are over 20%. An additional group of people who may have elevated levels are those with hemolytic anemia, a condition in which the body lyses some of its own red blood cells.
There are conditions that merit this test, even if carbon monoxide poisoning has not been definitively proven. These include symptoms of dizziness, nausea, headache, and irritability. If the blood carboxyhemoglobin levels indicate carbon monoxide poisoning has taken place, patients are typically treated with pure oxygen to facilitate the re-binding of oxygen to the hemoglobin molecules.
The carboxyhemoglobin that is found in the body naturally is produced from the decay of hemoglobin. An important component of hemoglobin is the heme group, a cluster of nitrogen molecules that binds the oxygen or carbon monoxide molecule. When this heme moiety is degraded, carbon monoxide is produced. At the levels produced, it acts a signal to transmit impulses from the nervous system. Researchers who discovered this process were surprised to learn that low levels of carbon monoxide have a function in cellular metabolism.